Air in power steering pump

[w113abudhabi]

Hello, I hope someone can offer some support.
My mechanic has installed a replacement power steering pump seal kit on my 1970 280sl. The kit is genuine mercedes purchased from the classic center. Prior to removing the pump the only issue was a fluid leak. The steering functioned well.

Now there appears to be a lot of air trapped in the pump resulting in no power steering at low revs. They have tried bleeding the air out of the pump following the mercedes manuals but still air remains.

Any suggestions please?

[Cees Klumper]

All I know to bleed air from power steering systems is to turn the wheels lock to lock with the engine running, until the air is purged. Not sure whether this is supposed to also work 100% on our W113's.

[MikeSimon]

Any P/S system will purge itself after a while, using the procedure Cees described. Some have a bleed screw to ease the process. The Pagoda steering gear has that. It sits on top towards the steering shaft input. Connect a hose to the nipple and stick the other end into the open reservoir on the pump. Open the nipple two full turns and let the air come out.
When done, close everything up, let the engine idle and turn the steering wheel several times end to end. repeat the bleeding to ensure all air is out.

[w113abudhabi]

This is exactly the process that has been followed repeatedly yet still air continues to be present and prevent any power assistance at low revs.

[MikeSimon]

Air in the p/s system will show itself by noise and/or a foaming fluid in the pump reservoir. If you do not have that, your power steering pump may be faulty, especially as you refer to "no support at low revs".
Air can be trapped at a high point in the circuit. Make sure hoses are properly routed and there is no high loop where air could sit. Other than that, it is, maybe, a matter of being persistent and keep bleeding.

[w113abudhabi]

Yep plenty of foam

[Benz Dr.]

That's because someone didn't read carefully back when I posted about this procedure. ( others may have as well )

 Do this with your engine OFF. A running engine will only churn all of that air into foam. Remove the cover on your canister and add ATF as required because it will lower as the air gets purged out. You really don't need to go full lock and side to side will work well enough until you can't see any more air coming out and then go full lock a couple of times. This may cause the tank to overflow so leave it a bit low until finished and then top up at the end.

Do this on well rested ATF without any hint of trapped air in it.

 

[MikeSimon]

Dan: How do you get the air out of the system without the pump running and pumping?

[Benz Dr.]

Dan: How do you get the air out of the system without the pump running and pumping?

When you turn the steering wheel it moves the steering box internals and forces any air out with the ATF. The trick is to lift the front of your car so the tires are not touching the ground so the wheel turns more easily. Trust me, it works like 10 men.

[MikeSimon]

I figured that this was the idea behind it. But this way, you only move the piston in the steering gear. Any air way down in the system and the hoses will not come out. If the bleed nipple is on the highest point in the system, you may have success.

[MikeSimon]

If continuing efforts to purge the air will fail, you may still have an issue with the pump.
While I worked for Vickers, all our power steering pumps, by design, had a bore that connected the space immediately behind the front shaft seal to the inlet.
This was done to "suck" the fluid that would accumulate behind the seal back to the inlet and avoid building up pressure in the area, which would cause leaks.
I don't know if the VT27 had that feature already, but I assume so.
The shaft seal is a double-lip seal, which seals towards the inside and the outside. If this seal does not sit right or the lips do not make proper contact with the shaft, the pump will "suck" air through the seal and you will continue to have air in the system.

[Benz Dr.]

I figured that this was the idea behind it. But this way, you only move the piston in the steering gear. Any air way down in the system and the hoses will not come out. If the bleed nipple is on the highest point in the system, you may have success.

It actually sucks fluid out of the canister as you move the steering wheel then it pushes it back into the canister when you turn the wheel in the opposite direction. During this process any trapped air will simply escape. I can only tell you that it works.

I've used the bleed nipple on the steering box with engine running and usually ended up with a mess.   

[MikeSimon]

Mmmmh….That will not make sense to me. If the engine does not run, the flow control valve inside the pump is in the "closed" position, which will effectively cut off the connection between the pump and the steering gear. I cannot see, how moving the piston in the gear will "suck" fluid out of the reservoir.
But if that method works for you, Dan, more power to you.
Sometimes "engineering science" does not have an explanation for everything.

[Benz Dr.]

Mmmmh….That will not make sense to me. If the engine does not run, the flow control valve inside the pump is in the "closed" position, which will effectively cut off the connection between the pump and the steering gear. I cannot see, how moving the piston in the gear will "suck" fluid out of the reservoir.
But if that method works for you, Dan, more power to you.
Sometimes "engineering science" does not have an explanation for everything.

OK, maybe I can't explain how it works, yet I know for a fact that it does. Why don't you try it and see for yourself? 

 I'm not talking about the  pump; I'm talking about the steering box, which when moved through the steering wheel WILL in fact push all of the air out. If your canister gets too low during this process you will indeed hear a suction sound so fluid is moving in that direction. 




 

[Aaron h]

Mmmmh….That will not make sense to me. If the engine does not run, the flow control valve inside the pump is in the "closed" position, which will effectively cut off the connection between the pump and the steering gear. I cannot see, how moving the piston in the gear will "suck" fluid out of the reservoir.
But if that method works for you, Dan, more power to you.
Sometimes "engineering science" does not have an explanation for everything.

Mike, the valve you make mention of is an over pressure valve, and will remain closed until hydraulic pressure from the locking of the steering wheel (full left lock or full right lock) opens it.  It's in place to prevent excessive pressure build up when in full left/right mode.  Otherwise oil would spew out of every O-ring and radial seal on the high pressure side of the pump and gear box.  What you call a "flow control valve" is actually a "pressure" valve, and is located inside of the steering gear box itself.  It is actuated by movement of the steering wheel.  It essentially directs oil under pressure to the front or rear side of the power piston inside of the steering gear box. 

Benz Dr.'s method of bleeding the system is, in fact, a very easy and fool proof way to bleed the system of air.  By moving the wheels left to right WITHOUT the engine running causes the power piston to literally suck oil into the system, and when moved in the opposite direction the displaced air escapes through the reservoir.  You must remember that the procedures listed in the BBB aren't always the best, correct, or easiest methods. The BBB isn't the law.....it's only a suggestion.

You mentioned that you don't understand how moving the piston in the gear will "suck" fluid out of the reservoir.  It's simple......imagine a syringe sucking a liquid up into it.  The only difference is in a syringe there is no air to displace, and if there is you simply turn it needle side up and depress the plunger until the air bubble has escaped through the needle.  An even better example is one of those under water water pumps you use in something like a koi pond.  When the system is new or has been serviced the water lines are full of air.  As soon as the pump starts moving water the water displaces the air in the lines until the lines are full of nothing but water.   Likewise, the more times the power piston in the steering gear box moves from top to bottom, and them from bottom to top, more and more air will be displaced.  Though, during the process keep the bleeder screw open and plug it off with your finger when the power piston is on the down stroke.  Otherwise it'll constantly suck air in.  On the upstroke of the power piston release your finger off of the bleeder screw.  When oil starts to emerge from the bleeder screw all/most air has been displaced.  Any residual will be displaced by running/driving the car.  Even if you didn't bleed the system at all the system would eventually displace all of the air on its own.  It would just take several miles, initially no power assist, and putting up with some groaning from the system until all of the air was displaced. 

There is ALWAYS a hydraulic connection between the pump and the steering gear box.  There is no valve that prevents this (except when in full lock left/right).  There are several ways to achieve bleeding the power steering system.  The methods listed in the BBB and by Benz Dr. are the easiest, cleanest, and less time consuming methods.  All methods lead to the same end result.  How you get there is your choice.  Just keep an open mind about how to approach things like this.  Never take the BBB seriously, and never overthink the process or try to improve upon it.

[Pawel66]

I am sorry to interfere here, again, ahead of the front line of my knowledge...

But I was working on the steering box of my W463 recently. I think the control valve is open, in essence, and it is closed in "far" positions by piston or something moved by piston to cut the flow in the "far" or lock position, to cut fluid/cut power to stop turning the wheel.

Am I correct or it is too late here already?

[Aaron h]

Well, there are actual mechanical stops to stop the wheels from turning on the spindles.

The control valve you make mention of......are you referring to the over pressure valve in the pump itself, or the actual pressure valve in the steering gear box? I have to assume from your commentary that you're referring to the over pressure valve.  If so, then I have to assume that it's always closed since it's under spring pressure upon installation, and opens upon over pressure, which allows fluid under pressure to bypass the steering gear box and return to the reservoir.  Then again I could be very wrong......I often am! 

[Benz Dr.]

Mike, the valve you make mention of is an over pressure valve, and will remain closed until hydraulic pressure from the locking of the steering wheel (full left lock or full right lock) opens it.  It's in place to prevent excessive pressure build up when in full left/right mode.  Otherwise oil would spew out of every O-ring and radial seal on the high pressure side of the pump and gear box.  What you call a "flow control valve" is actually a "pressure" valve, and is located inside of the steering gear box itself.  It is actuated by movement of the steering wheel.  It essentially directs oil under pressure to the front or rear side of the power piston inside of the steering gear box. 

Benz Dr.'s method of bleeding the system is, in fact, a very easy and fool proof way to bleed the system of air.  By moving the wheels left to right WITHOUT the engine running causes the power piston to literally suck oil into the system, and when moved in the opposite direction the displaced air escapes through the reservoir.  You must remember that the procedures listed in the BBB aren't always the best, correct, or easiest methods. The BBB isn't the law.....it's only a suggestion.

You mentioned that you don't understand how moving the piston in the gear will "suck" fluid out of the reservoir.  It's simple......imagine a syringe sucking a liquid up into it.  The only difference is in a syringe there is no air to displace, and if there is you simply turn it needle side up and depress the plunger until the air bubble has escaped through the needle.  An even better example is one of those under water water pumps you use in something like a koi pond.  When the system is new or has been serviced the water lines are full of air.  As soon as the pump starts moving water the water displaces the air in the lines until the lines are full of nothing but water.   Likewise, the more times the power piston in the steering gear box moves from top to bottom, and them from bottom to top, more and more air will be displaced.  Though, during the process keep the bleeder screw open and plug it off with your finger when the power piston is on the down stroke.  Otherwise it'll constantly suck air in.  On the upstroke of the power piston release your finger off of the bleeder screw.  When oil starts to emerge from the bleeder screw all/most air has been displaced.  Any residual will be displaced by running/driving the car.  Even if you didn't bleed the system at all the system would eventually displace all of the air on its own.  It would just take several miles, initially no power assist, and putting up with some groaning from the system until all of the air was displaced. 

There is ALWAYS a hydraulic connection between the pump and the steering gear box.  There is no valve that prevents this (except when in full lock left/right).  There are several ways to achieve bleeding the power steering system.  The methods listed in the BBB and by Benz Dr. are the easiest, cleanest, and less time consuming methods.  All methods lead to the same end result.  How you get there is your choice.  Just keep an open mind about how to approach things like this.  Never take the BBB seriously, and never overthink the process or try to improve upon it.

I keep the bleeder screw closed - cuts down on the mess. BTW, you're hired!

[Pawel66]

Well, there are actual mechanical stops to stop the wheels from turning on the spindles.

The control valve you make mention of......are you referring to the over pressure valve in the pump itself, or the actual pressure valve in the steering gear box? I have to assume from your commentary that you're referring to the over pressure valve.  If so, then I have to assume that it's always closed since it's under spring pressure upon installation, and opens upon over pressure, which allows fluid under pressure to bypass the steering gear box and return to the reservoir.  Then again I could be very wrong......I often am! 

I thought I talked overflow valve in the steering box. Then I go t it wrong. Thank you and sorry for messing around.

[MikeSimon]

Sorry, guys. I worked for the manufacturer of the W113 power steering pump - Vickers in Bad Homburg, Germany
I appreciate all the opinions of what the p/s system does and how it works, but I was actually paid for knowing that.
The pump has a valve, which is a combined flow control/pressure relief valve.
The flow output of the pump is direct proportional to the pump speed, i.e. rpm of the engine.
The pump would put out an excessive amount of fluid at high rpm and /or cavitate. That is why it has a "flow control valve"
This is a spool, which sits inside a bore. It moves horizontally and allows  flow to the pump discharge port until its horizontal movement opens an orifice inside and thus limits the flow at that point.
The spool also has a check valve inside which works against a calibrated spring and opens at a certain back pressure. When a maximum pressure is reached the check valve opens and bypasses output flow thus limiting the pressure in the system preventing seals and hoses from blowing up.
There is only one other valve in the system, and this is the steering valve actuated by the steering wheel/column. This is a rotating sleeve valve which directs flow to either the right or left side of the piston in the steering gear creating support.
Hydraulics 101: No pump creates pressure. A pump creates only flow which in turn, depending on the resistance and diameters of the output creates a pressure. If you disconnect the "pressure hose" from the pump to the steering gear and rev the engine up, you would just see fluid splashing out. The pressure relief valve in the pump would not cut in, no matter how high the engine revs or how far you turn the wheel. If you connect everything, you will hear the pressure relief valve "hissing" when you turn the steering wheel to a stop, thus preventing the piston in the gear from moving further and the pump "working against a wall". A safety feature.
All bypasses are happening in the pump.
There are steering systems that have an additional relief valve in the steering gear/box, but the Mercedes gear is not one of them. This is done when engineers try to prevent the pump from overheating by recirculating flow at pressure relief internally. You should never hold the steering wheel at wheel lock (end of turn) for longer than 30 seconds. You will "fry" the pump.

[Benz Dr.]

Sorry, guys. I worked for the manufacturer of the W113 power steering pump - Vickers in Bad Homburg, Germany
I appreciate all the opinions of what the p/s system does and how it works, but I was actually paid for knowing that.
The pump has a valve, which is a combined flow control/pressure relief valve.
The flow output of the pump is direct proportional to the pump speed, i.e. rpm of the engine.
The pump would put out an excessive amount of fluid at high rpm and /or cavitate. That is why it has a "flow control valve"
This is a spool, which sits inside a bore. It moves horizontally and allows  flow to the pump discharge port until its horizontal movement opens an orifice inside and thus limits the flow at that point.
The spool also has a check valve inside which works against a calibrated spring and opens at a certain back pressure. When a maximum pressure is reached the check valve opens and bypasses output flow thus limiting the pressure in the system preventing seals and hoses from blowing up.
There is only one other valve in the system, and this is the steering valve actuated by the steering wheel/column. This is a rotating sleeve valve which directs flow to either the right or left side of the piston in the steering gear creating support.
Hydraulics 101: No pump creates pressure. A pump creates only flow which in turn, depending on the resistance and diameters of the output creates a pressure. If you disconnect the "pressure hose" from the pump to the steering gear and rev the engine up, you would just see fluid splashing out. The pressure relief valve in the pump would not cut in, no matter how high the engine revs or how far you turn the wheel. If you connect everything, you will hear the pressure relief valve "hissing" when you turn the steering wheel to a stop, thus preventing the piston in the gear from moving further and the pump "working against a wall". A safety feature.
All bypasses are happening in the pump.
There are steering systems that have an additional relief valve in the steering gear/box, but the Mercedes gear is not one of them. This is done when engineers try to prevent the pump from overheating by recirculating flow at pressure relief internally. You should never hold the steering wheel at wheel lock (end of turn) for longer than 30 seconds. You will "fry" the pump.

So, what are we actually talking about? I thought it was simply bleeding air out of the system.

[450sl]

If you connect everything, you will hear the pressure relief valve "hissing" when you turn the steering wheel to a stop, thus preventing the piston in the gear from moving further and the pump "working against a wall". A safety feature.



Mike, i hear no hissing but a loud Screaming sound when turned to a stop ,  (did swapped the pump but to no avail =same noise).

Can you advise?

thanks
Mark

[Benz Dr.]

Is your PS belt tight?

[MikeSimon]

Mark: Noise in a power steering system is a very complex issue. The pressure relief valve just produces a "hiss" caused by pressurized fluid blowing by inside the pump. But that in turn can create a lot of other noises caused by vibration transferring through the hoses and tubing. Depending on what car you have, metal tubes in the system should be isolated from the body through rubber grommets. The steering gear itself can create noise when the piston vibrates. Of course, like Pawel mentioned, the belt can also be a source of noise, but I ruled that out as you said you changed the pump and I assume you checked belt tension. My guess would be you have airborne noise coming from a tube or hose.

[Aaron h]

Also Hydraulics 101:  If a pump creating fluid flow moves fluid that has nowhere to go then the resultant is fluid under pressure acting upon all surrounding surface areas within the system.  I never said or eluded to any suggestive commentary that would lead one to believe that a pump creates pressure, directly or indirectly. 

You stated earlier, "I cannot see, how moving the piston in the gear will "suck" fluid out of the reservoir".  If after explaining to you why moving the piston DOES take fluid out of the reservoir you still can't comprehend then I guess there isn't much we can do to convince you.  It may not take the fluid out of the reservoir on the high pressure side.....it likely takes it in on the return side with the down stroke of the piston.  The fact remains that no matter how the fluid is taken in it still displaces air and achieves the intended goal.  With your knowledge you should be able to make sense of this, especially after your having further elaborating on what I had already stated.  I feel that if you'll keep an open mind and just try one of the aforementioned methods you'll see that what was suggested does work....  So well, in fact, that if done properly you can literally start the car an go with 100% power assist right away.  Still don't believe it?  Well, again, try it!  I learned a long time ago that the physical properties learned in the mechanics of fluids classes is pretty black and white, but the manipulation of fluids is not.....and this is exactly why there is more than one way to bleed this particular hydraulic system, be they conventional or not. 

And yes, this post is about bleeding the power steering system.  The original poster is probably very lost right about now.  So I'd like to apologize to the original poster for having hijacked his thread.   Back you you, Abudhabi. 

[Benz Dr.]

Also Hydraulics 101:  If a pump creating fluid flow moves fluid that has nowhere to go then the resultant is fluid under pressure acting upon all surrounding surface areas within the system.  I never said or eluded to any suggestive commentary that would lead one to believe that a pump creates pressure, directly or indirectly. 

You stated earlier, "I cannot see, how moving the piston in the gear will "suck" fluid out of the reservoir".  If after explaining to you why moving the piston DOES take fluid out of the reservoir you still can't comprehend then I guess there isn't much we can do to convince you.  It may not take the fluid out of the reservoir on the high pressure side.....it likely takes it in on the return side with the down stroke of the piston.  The fact remains that no matter how the fluid is taken in it still displaces air and achieves the intended goal.  With your knowledge you should be able to make sense of this, especially after your having further elaborating on what I had already stated.  I feel that if you'll keep an open mind and just try one of the aforementioned methods you'll see that what was suggested does work....  So well, in fact, that if done properly you can literally start the car an go with 100% power assist right away.  Still don't believe it?  Well, again, try it!  I learned a long time ago that the physical properties learned in the mechanics of fluids classes is pretty black and white, but the manipulation of fluids is not.....and this is exactly why there is more than one way to bleed this particular hydraulic system, be they conventional or not. 

And yes, this post is about bleeding the power steering system.  The original poster is probably very lost right about now.  So I'd like to apologize to the original poster for having hijacked his thread.   Back you you, Abudhabi.

Just imagine Aaron. 10 hour drive to get here and all the snow you can eat.

[450sl]

Thanks Mike, will do the engine off -stop to stop procedure, and check my tubings.

Cheers
Mark

[w113abudhabi]

Thank you for all the responses and apologies if I have started a difficult dialogue.

I have shared all comments with my mechanics who have successfully removed all air from the power steering system. The pump has been in and out the car several times in the past weeks and issues remain. I will summaries below some key points:

1. The power steering was functioning well but the pump had fluid leaks
2. An original mercedes seal kit has been used to prevent the leaks
3. There are no visible signs of wear or damage to the pump whatsoever
4. Following advice from this forum it is believed that all air has been purged from the system
5. Whilst removed from the car the pump has been pressure tested and holds good pressure

However, there is no power assisted steering at below 1200 rpm. Once the revs exceed that level the steering is power assisted.

Can anyone please shed any light or suggestions?

[MikeSimon]

There are obviously  several more people here who know more about the power steering system than I do, and I am sure, they will chime in. In the meantime, I will make a comment.

The flow output of the power steering pump, as I said, is direct proportional to the rpm. That is the typical characteristic of the balanced vane pump.
If one would plot the output of the pump over rpm, you would see a rising flow until a certain rpm when the flow control point is reached. From there up in rpm, the flow remains basically constant, regardless of the back pressure.
In the lower rpm part, the "non-controlled flow" part, the output is sensitive to back pressure. If the back pressure increases (steering the wheels) the flow goes down. When designing a p/s system, the "dry park" condition is the key for picking the displacement size of the pump. At engine idle and back pressure at the parking maneuver, the pump has to put out enough flow to provide the torque in the gear to steer.
If everything is in proper condition as you describe, there could be two reasons the car will not steer;
1- the pump displacement is too small ( I want to rule this out as you said the pump was rebuilt and it is probably unlikely they put the wrong size rotating group in) -OR-
2- the internal leakage of the pump is too large. This could be caused by excessive wear on the sides of the rotating group, a missing vane, or excessive wear in the valve bore.

The proper diagnosis for the pump will be to test it for flow at low rpm and a back pressure of, let's say, 500-600 PSI (dry park condition)
If memory serves me well, you should get around 8-10L per minute (2.5-3 gpm). The pump building pressure at high rpm is not a sign for a good pump.
Most service dealers when hydraulic p/s was popular in cars had a simple test set-up for the pump. It consisted of an extra line with a flow control meter, pressure meter and a shut-off valve.
This line was connected in the pump output line between pump and gear. Then the back pressure was increased by slowly closing the shut-off valve until the pressure meter would indicate the desired back pressure. The reading of the flow meter would give an indication of pump condition.
If you do not get the required flow, the pump needs to be opened up and checked again.
Any hydraulic service shop can test that pump for you. Probably cheaper than the Mercedes dealer

Now take all this with a grain of salt and maybe wait for better explanation because as stated in previous posts, I do not have the right comprehension regarding these issues.
Also, we can take this off the forum, if you like to at mike.simon70@yahoo.com

[Cees Klumper]

At this point I would try installing a good used replacement pump.  They can't be that expensive, off Ebay or what have you.

[Aaron h]

Mike, it wasn't the intent of any of us to imply that you hadn't the ability to comprehend anything regarding the material in this post.....you're more than qualified, and we all recognize that.   All we were trying to do is prove to you that it is possible to rid the system of air without having to run the engine and pump.  The method Dr.Benz made mention of is a tried and true method that Mercedes dealers and private shops across the world have used for 50+ years without fault.  All we had hoped for was a little understanding and for you to give it a try.  Nothing more, nothing less. 

Regarding abudhabi's issue, we first need to make some corrections to some of the numbers and devices used for testing.  We'll first touch on flow rates for the Vickers winged cell pumps.  (Refer to pictures below) The pump used on Pagodas had a flow rate of 5.6 liters per minute (about 1.47 gallons per minute) at 500 RPM.   8-10L per minute seems really high.....or was that assuming there was no flow control or counter pressure in which the pump was moving fluid at it's maximum RPM?.   I realize that in the reference picture it denotes a W108.015, W109.015, and a W109.016. However, the superscript "1" is a foot note that states "And all other types optionally equipped with DB power steering.".  Obviously, that would include the W113.

For counter pressures, we need to know if the pump is on a test stand or operating on the car....two very different operating/test conditions.  The early BBB notes a TSB from Vickers that says "At a counter pressure of 50 ATM (735 psi) and at 500 RPM the minimum delivery of the pump should be 5.7 liters per minute (1.5 gallons per minute). However, these values can only be measured on a test stand."  In the later BBB Mercedes took this into account and revised the circulating pressure testing procedure to reflect the tests of the pump being done with the pump on the car.  Though, in the early book Mercedes still shows how to perform tests on the pump with it still on the engine and connected to the gearbox.  This brings us to the next item of interest......the test jig. 

The testing device only consisted of a throttle valve and a pressure gauge.  The pressure gauge gives us the pressure readings, and the throttle valve acts as the the counter pressure. There was an early and a late set up of the test jig. (see pictures below).  Both achieved the same result.  However, there was no flow control meter, as it wasn't necessary due to the pressure gauge being present and in use throughout the entire test, as short as the test was.  Though, a flow control gauge would be nice to integrate for peace of mind.  Regardless, it was a simple set up.  The hydraulic line going between the pump and the gear box was removed at the pump.  The gauge side of the jig was connected to the pump, and the throttle valve side of the jig was connected to the end of the hydraulic hose that was removed from the pump. 

After the oil was topped off, the throttle valve was opened, the engine was started and circulating pressures were noted from the pressure gauge at idle speed (700-800 RPM).  Circulating pressure should NOT exceed 4 atü (71.1 psi).  During this time the steering wheel is not to be turned, as the control valve in the gear box would be displaced causing pressure to increase.  Next, the throttle valve was closed to measure the maximum pressure of the pump.  This was done by revving the engine up to 1,000 RPM and holding it there while noting the reading on the pressure gauge.  The maximum pressure should be at LEAST 55 atü (796.5 psi), but should NOT exceed 65 atü (938.7 psi).  Alternatively, maximum pressure can also be measured by turning the steering against left hand and right hand lock.  Regardless of which test for maximum pressure is chosen, make the test as quickly as possible, as running the pump too long with the throttle valve closed or steering locked in one direction or the other can cause the oil to get too hot and cause potential damage to the pump itself.  Also, note in the pictures below that some of the pumps have the maximum pressure printed on their identification tag. 

So, to review all of the aforementioned in short:

-Flow rate of pump is 5.6 liters per minute (1.47 gallons per minute)
-Circulating pressure should not exceed 4 atü (71.1 psi)
-Maximum pressure should be at LEAST 55 atü (796.5 psi), but should NOT exceed 65 atü (938.7 psi)

Oddly, none of the literature gives us a minimum circulating pressure value.  Mike, in your professional opinion, what would you consider a minimum circulating pressure value to be?  Or, what minimum value do you think would deem the pump faulty? 

[MikeSimon]

Aaron: The flow rate for the pump at 500rpm is a Vickers pump spec and was given to determine the correct displacement. It is used on a test stand. The engine idle in a car is not 500 rpm. I based my flow rate on an engine idle of 850-1200 rpm to test the pump at real world conditions. The VT27 used by Mercedes Benz in early p/s was, I believe, a 13cc unit. As the flow is direct proportional to rpm, following the Vickers spec at 1000 rpm, it would be 11.2 liters. (Theoretical flow would have been 13L/min)
Pumps have an internal leakage, so mathematical calculations are not always 100% correct. As I said, back pressure affects the pump performance below flow control greatly.
The required flow rate of the pump for a steering system is based on the flow required to achieve 1-1.5 steering wheel turns per second. It is not the same as the 5.6L given as the spec number.
As far as the circuit pressure at closed steering valve (straight ahead) is concerned, for layout purposes we always assumed it to be between 30-50 PSI, depending on the length of the hose routing and the size of the steering gear.
Vickers always stamped the maximum pump relief pressure on the nametag of the pump. This is the pressure value the internal relief valve is set at. It can vary for the same pump model depending on the application.
As far as the p/s test kits are concerned there are a few different ones around. I am not familiar with the one MB used at their dealers, but we used one with a dual meter, flow and pressure.

Interesting side note, as we have the "different world words thread" elsewhere: The pump in the Mercedes Benz literature is called a "winged  cell pump". This is a literal translation of the German term "Flügelzellenpumpe". Although Vickers - being a US company - used the proper English term "Vane Pump"

[w113abudhabi]

Problem solved. Thank you to all for their help and recommendations.

It turned out that the control valve had been reinstalled in the pump the wrong way around resulting in power steering only being available when the revs exceeded 1000rpm.

The following linked thread and attached file provided the necessary information.

https://www.sl113.org/forums/index.php?topic=20219.0

Thanks again!

[Cees Klumper]

Congratulations, a very simple cause. And we got to watch an intellectual debate between experts to boot!

Must say when I read a post in the thread you referenced, where Johnny quoted me as having contributed probably 10+ years ago on this subject of bleeding the power steering pump, I have zero recollection of that.