Main.TrailIndexPage | Brake System | Brake.Booster

Brake Booster

This component is part of Brake System.


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When I bought my '66 230 SL last month I knew it had a break problem. I first took it to an independent mechanic who replaced the expansion cylinders for the rear shoes (one was leaking). They also completely flushed the fluid from the whole system and refilled. Since then, my breaks went from weak and soft to acting like manual breaks. I get about 3 inches of free play and then a very firm pedal. At that point the breaks work well (no pulling, etc.), but they take CONSIDERABLE effort. I pulled the vacuum hose leading from the intake manifold to the break booster and it has vaccuum. When the hose is open, the engine revs. I am not losing break fluid from the reservoir above the master cylinder. (1) Could it be anything other than the break booster? (2) If it's the booster, can it be repaired instead of replaced ($425)?

Will Samples says: It certainly sounds like a bad brake booster. The booster is not repaired, it is replaced. I am sure it can be repaired, it is just that ATE never sold any parts to fix it. Another thing to consider is the offset bolt that connects the booster to the lever on the firewall. Unless yours has been changed, the bolt is an eccentric one and has a notch in the head. This notch should point to the rear, which removes as much pedal free play as possible. I can also get you a booster for a little less than the $425.

Recently I have noticed the idle speed increases by 100-200 rpm when I release the brake pedal after waiting at a stop light in neutral. The car is a 4 speed manual. Is this normal, or does it indicate perhaps a vacuum leak or some other problem?

Your problem could well be that the idle speed solenoid (that is, if your car is an automatic or has airconditioning), located above the intake manifold, is not functioning (properly) anymore - this happens a lot. Check the other posts on this topic, there have been many. Contact me offline if this appears to be the problem, I have more experience with this item than I care to think about.

Thanks Cees, however I don't have an idle speed solenoid. I suspect this is a problem with the brake booster, or a leak in the brake booster hose. Today I tried disconnecting the brake booster hose and plugged the hole in the intake manifold where it connects. The car idles smoother and the idle speed is now constant when the brakes are applied. Hopefully I don't need a new brake booster.

The vacuum assist on my 65 230SL has ceased working. The brakes barely work although the car will still stop with sufficient force applied. Is there an easy way to troubleshoot the system? There is a check valve in the vacuum line between the air intake manifold and the brake booster. I have a leaky rear wheel cylinder and I'm not sure if that would cause the problem. I am trying to fix that as well. Also, are there rebuild kits available for these or do I have to shell out the $425 for a new one.

I am definitely not an expert. I have a 230 too. A year ago my brake booster (whatever the big-black-round-"box" you have in front of the driver within the engine bay is called) progressively lost power, and car took more and more pressure from my right foot (manual car) to stop. After a while it had been going on I thought it was just an impression, since it effectively stopped when and where I wanted to. But it evolved into unreasonable strenght and pedal displacement. If this is your case too, there is no other fix than changing the "box" as soon as you can. Tom Hanson can provide you with the code for the piece. In my case (based in Europe) they supplied me from Germany with a 280 piece, slightly larger than the original one, and it even feels that brakes are better than ever before. I guess it will be the case all over the world. Changing it is easy, and it is kind of impossible, and not at all recommended trying to repair the old one. Too much of a risk.

I have the same symptoms (hard pedal to push), and I spent about $450 for an ATE brake booster. I believe it's correct for the 230SL. I have not installed it yet. While spending the money, you might as well get a new check valve, new vacuum hose, and new hose clamps. They are very inexpensive compared to the booster and will give you peace of mind. You can not repair/rebuild these boosters. Before doing the booster, I would fix any leaks. Luckily, my system has not leaked since I got a rear brake cylinder repaired. Did you replace the booster yourself? Any tips?

I replaced my brake booster about two years ago. Please pay special attention to the bolt which connects the brake pedal assembly to the booster. In addition to this bolt and nut there should be a bushing which will not allow you to tighten the pedal assembly and booster brackets together. There is slight rotation in the brake pedal assembly at this connection when the brakes are applied. The brake booster bracket should be permitted to spin free of this rotation because of the bushing. If this bushing is not in place the slight rotation of the brake pedal assembly will cause the vacuum seal to open which decreases pressure and increases the force needed to stop the car. Took me a couple of days to think through the issues before I discovered the problem. I hope the explanation is clear enough.

I recently noticed that pushing the brake pedal at idle causes my engine speed to increase by 100-200 RPM. Suspecting a vacuum leak in the brake booster, I removed the brake vacuum hose from the intake manifold and plugged the intake hole with a rubber ball. The brakes no longer affect the idle speed. Does this mean I need a new brake booster? Is there another test I should do to confirm the booster is in fact faulty? Are booster rebuild kits available?

Tom Hanson says: MB doesn't supply kits for these boosters. Maybe there is some aftermarket alternative. I have access to them direct from A.T.E at a decent price. Check the booster hose carefully.

When I come to a stop, and apply the brakes, the rpms increase up to 1500-1600. When I release the brake the rpm drops nicely to the 800-900 range again. Note that this is a 4 speed stick shift, not an automatic. I even tried to lift the accelerator pedal to see if somehow the brake is pressing on the accelerator somehow, but it didn't drop the rpms. Hmmmm... any takers?

Sounds like you have a vacuum leak in your brake booster. To test, remove the booster hose where it connects to the intake manifold, and plug the resulting hole in the manifold. The idle should now remain constant when the brakes are applied. Reattach the hose before driving the car to restore the power brakes.

I measured the Intake Manifold Pressure off of the brake booster fitting (on the manifold) the other day. It was at 12 lbs. My compression is kind of low too (~125 lbs each cylinder, but it was measured on a relatively cold engine). What are some of the possible causes? Bad Valve adjustment, old tired engine (it still goes pretty good though and doesn't burn much oil, although oily smoke comes out of the dipstick when I check the oil with the engine running), leaky manifold (in this case, how do I pinpoint the leak), or is this the right number. I know that the brake booster uses this manifold pressure to assist the braking process, so I figure if this is too low then the brakes won't be as good as possible. On another note, can any of you guys/girls completely lock up the wheels when braking hard (especially the front wheels). I have never been able to do that, but currently my booster is either slightly defective and/or I have a leak in the vacuum lines...

I have blocked them twice, luckily with no consequences. Once with a lot of noise and smoke, thanks to a truck and a pedestrian who crossed the street. I was impressed by the handling of the car (straight line, though, not in a turn. I did not have such experience and do not look forward to), and also by the performance of a new booster I had installed a month before. Therefore my opinion is to go for booster replacement as soon as you think it may be a good thing the first time, and not wait a montho or a year...

Claus Stetter: I am barely able to lock up the brakes on my 230SL. Mechanic says the brakes are fine but it still is not at all up to par to the 4-wheel discs on my 6.3. The rears lock up easier than the fronts - maybe to do with the weight shift to the front?

For safety reasons, front should lock up first. It could be quite dangerous the other way round (sideways driving thrill!). Moreover front has always discs on w113, and rear drums.

Booster of not, with enough foot pressure you must be able to lock the wheels. If not, I suspect hydraulic problems or grease on the discs or something.

I'm looking for a brake booster. Is the T51 ATE the right model. I found one with the following numbers ATE 3.6140-2100.4, with T51 stamped into another part of the booster. It looks somewhat smaller (in the picture) than the one in my sl. Any experts out there know if this would be the correct one? P.S. Boy, are they expensive!

Apparently the 230 specific ATE model is not sold anymore (or at least I was told so). I got mine replaced, from SLS-Germany supplier, with the ATE booster that goes to 280 SLs, which is supposed to be more powerful and fits nicely, but it is bigger than the 230 oginal model.

Bernt Damm: I don't have the specific numbers but the earlier MBs like the 230 had the double membrane brake booster which I think is T51/2. It is thicker than the later ones which are single membrane. The later ones give far less assisting power than the older ones. MB stopped using the older one because drivers had a feeling of complete brake loss when the booster packed in. This was because the assistance was so strong that the difference in pedal pressure between assist and non assist was so great that drivers would panic. I am bashing this out from memory but anyone with a MB service manual can confirm it because that is where this info comes from. So, the later brake booster is weaker and smaller because it only has one membrane in it instead of 2. It requires harder pedal pressure but will give less of an effect of 'total' brake loss in case the booster gives in.

What's the number stamped on the side of your booster. Did it fit ok? The T51 is supposed to be the model that is used on the 280SLs. Bernt Damn also had an interesting comment (T51-2). I don't mind going to the single diaphram booster as long as it fits. Especially since it might not be available anymore.

Albert: I replaced the brake booster of my '64 230 SL two years ago because the old one had leaks. On my new brake booster you can read:

 And in the blue sticker: 
 Ate 3.6197-0220.4 244 6 A4S

I don't know if it's like the original (it seems not to be available), but it works fine for me. I'm happy with the brake pedal feeling. When the replacement, I needed to install also a valve in the pipe between the intake manifold and the brake booster. Otherwise the idle was not stable.

The brake booster on my 230SL is acting up. It's fine on the first application of the brakes, but loses its boosting ability on subsequent braking. If the brakes aren't used for a few minutes, again the first braking is fine. It holds vacuum well and the line to the booster supplies vacuum properly. Any idea what might be wrong and where is a good source for a replacement booster? Thanks.

There is a place that will rebuild them for $295 (exchange), New from star quality I was quoted $650 (or maybe it was $550). Check your booster check valve, it might be defective. Or maybe your brakes are misadjusted, which never allows the vacuum to build up properly. If the brake is partially activated, it might just be causing the symptoms you describe...

I think there are two types of Brake boosters: one works with this separate valve in the pipe between the intake manifold and the brake booster, and the other type has the valve in the brake booster itself. To check if you need the separate valve in the pipe, you can do the following test: With the engine on, press the brake pedal, and then stop the engine: if you observe a pressure of the brake pedal trying to go up, then you need the valve. This is because this valve is to keep some braking effect in case of engine stall.

I think I'm finally going to take the time and replace my brake booster. My first "winter project". I will look through the MB Service Manual and Haynes Manual tonight for instructions.

  1. How many hours should I expect this to take? Consider I have never done this before on any car.
  2. What precautions should I take? I want to prevent breaking other parts during removal/repalcement. Also, should I buy any other parts in expectation that they should be replaced? Any tips from you experienced people will be appreciated.

I replaced my brake booster 2 years ago. The process is pretty straight forward. The only problem I had was with bolting the booster to the brake pedal mechanism. It is very important that this connection is done properly. If not, depressing the brake pedal will cause the air pressure seal in the booster to fail which defeats the vacumm assist. It took me a while to figure this out. The brake pedal mechanism must swivel when bolted to the booster. My old booster had a bushing which prevented tightning the bolt and creating a fixed connection. The new booster was a little different. Hope this helps.

The booster is easy to do. Remove the air vacuum hose to the manifold from the booster housing end. Remove 4 13mm nuts from the back of the booster to the aluminum frame that's bolted to the firewall. Take out 2 13mm nuts holding the master cylinder to the booster in front. Romove the single large bolt holding the push lever in the back to the booster pin. Undo all of the connections, move the master cylinder forward and to the side to clear the pin inside the booster, then slip the booster off from the back and lift out. The large single bolt in the back has an offset surface which slots into the opening for the pin on the booster, and it will never tighten up or sieze as there is a spring loaded washer and a 13mm nut that holds it in. Tightening the nut will not have any impact on it. The offset surface of the bolt is to adjust the boost power. Hope this helps, should not take more then an hour for the DIY'er.

David Pease: I had never touched a brake booster before, but needed to replace mine. I read the BBB instructions several times, to understand the connections, etc., then proceeded to completely ignore those directions and do what Walter suggested (that is, not to dismantle the master cylinder, but simply move it carefully aside). I believe I spent an hour or so reading and thinking, then the job took no longer than half an hour. (I had told my wife I'd be outside for the whole afternoon, so was she surprised when she came out and found me finished!).

Here's how to test your booster: pump the pedal a bunch with the engine OFF to deplete any stored vacuum. Then hold moderate pressure on the brake pedal and START the engine. With a good booster, the brake pedal will go down farther when the engine starts. Shut OFF the engine while holding steady pressure. The pedal should stay where it is. If it doesn't go down or stay down, there is a problem with the booster or the check valve in the vacuum line.

How does one simply "move aside" the master cylinder? Are the brake lines flexible enough to allow this movement? How much movement will I need? There should be no fluid loss, right? Are there any other parts (connectors/washers/nuts/seals/etc.) that are typically replaced when the booster replacement is done? I've already replace the hose and check valve to the intake manifold.

As I remember, the only other parts I replaced were the vacuum hose, check valve, and inline plastic "sump" (I don't remember what this is really called). (Routing the new vacuum hose was what got me to notice the missing hose support that we discussed earlier.) In my case, I needed several inches of movement, enough to clear the pin that exits the booster and pushes into the master cylinder. I did my removal in a different order than Walter suggested; perahps his way you'd need less movement. The brake lines are somewhat flexible, though, of course, you need to be careful not to kink them. I was able to slowly and carefully move the master cylinder forward and slightly down (in an arc) enough to clear the pin. As I said, this is not the recommended way, but it's sure a lot easier than draining the master cylinder, detaching all the lines, reattaching and refilling, then bleeding the brakes, etc.! Yes, there should be no fluid loss.

I am finally working on replacing that brake booster. I have the master cylinder off and the 4 bolts off that hold the booster to the bracket. I also have the plunger disconnected from the brake pedal. Problem: I can't get the booster forward and out of the bracket because the plunger shaft is too long. The booster is running into the master cylinder again. Do I need to move the master cylinder completely out of the way? Forward or to the side? That seems like too much bending of the brake lines either way? Is it safe? How much do metal brake lines cost and are they a pain to install?

Bernt Damm: Drain the system through the front and rear brakes and then unscrew the lines. Remove the cylinder completely. That is safer and easier.

There is no need to worry about bending the lines too much. They only break when there is a kink in them or at the extreme bend areas. If you are still hesitant about it, you can remove the aluminum bracket on the firewall with 3 13mm bolts or nuts from under the dash inside the car. There are two on top and one on bottom, if I remember correctly. Remove the single bolt holding the pedal to the rod. Then remove the bracket from the firewall in the engine compartment and you have it easy. It's much simpler to pull and push the MC to the front of the car and to the side, you can twist it sideways too to get enough clearance to pull out the booster. Either way it works and the difference is amount of work. They are a little difficult to replace as they come from the dealer in straight pieces, you have to put all the bends and curves into them. Of my 8 MB models owned only 1 needed new brake lines(rusted out), and almost all needed new MC and or boosters.

On these cars, I have only removed everything for restoration, but on other cars you usually do have to carefully force (and slightly bend) the brake lines if you want to avoid leaking any fluid. Replacing the brake lines with everything installed would be quite a pain. (More than the heater controls).

OK, I did it! The lines bent and did not kink or crack. Everything went back together fine and I took it for a test drive. All I can say is WOW! What a HUGE difference! No more muscle cramps (or using neutral) at red lights. And now I'm going to have to get a set of those progressive rate springs to reduce the nose dive. I had that brake booster laying around for quite a while, but thought I had to take the master cylinder off the brake lines. I'm always worried about breaking the stuff connected to what I'm working on, so I figured I would be in for new brake lines and who knows what else. This method worked and was fast, but it was risky if the brake lines are bad. Of course, if they are bad, it's better that they break in the garage than on the road. Anyone want an old booster? I don't think these ATE T-51 boosters are rebuildable, are they?

Is there any way to test to see if a brake booster (230SL) is bad? Or perhaps I should ask what else could cause the brakes to require a lot of pedal pressure? In the archives I couldn't find any test procedures. There are a number of messages from owners whose brakes worked much better after replacing the booster, but that's expensive, and I don't want to do it unless I have to.

There apparently is an adjustment that can be made to the excentric bolt that connects the pedal to the booster. The bolt has a notch in the head. This notch should point to the rear, which removes as much pedal free play as possible. Also you should check the vacuum hose. In an earlier post, Joe Alexander noted to "make sure your rubber brake hoses are good. Check the back hoses also. There are a total of five in all. They make a big difference in brake responce, and performance. Make sure your brake fluid is fresh, the fluid collects moisture with age." Of course, you should make sure there's no air in the system. But this would be evident from the pedal response.

Having just gone through this I can offer you these ideas. First disconnect the vacuum hose from the booster. Hold your finger over the end of the hose and have someone start the car; is there vacuum there? Re-attach the hose start the car and shut it off remove the line from the booster. Do you hear vacuum escaping(or rather air being drawn in to the vacuum)? If so your booster is not leaking. If you do not hear it try reversing the vacuum hose and repeat test one. If the hose flows air in both directions then the check valve in the hose is bad.

You can also disconnect the vacuum hose at the booster, and then apply a vacuum with one of those little hand-pumps (MightyVac?). See if there are any air leaks.

An easy way to check to see if the booster diaphragm is good is to try to get a vacuum using your mouth. If the diaphragm has a hole in it you will just keep on sucking air out.

Rodd: here's another way to see if air is getting sucked in through the booster.

  1. Drive car until it is warmed up.
  2. Park the car and apply hand brake.
  3. Remove it from any gear.
  4. Apply foot brake and see if RPM goes up.
  5. Listen for a slight hisssing sound.

If RPM goes up and you get the hissing sound, it is because the vacuum that is being created by the engine in the intake manifold is pulling on the brake booster diaphram. The diaphram is ruptured and is letting air past. This air makes the hissing noise and it increase the air getting mixed in with the fuel, which raises the RPM. With new fluid, booster, front rotors, pads, caliper pistons, piston dust covers, piston seals, and rubber hoses, my car brakes like a new Benz!

Brake Booster Eccentric Adjustment: The eccentric bolt at the lower end of the cast bracket that holds both the actual booster body and the booster arm has a slot in at (for a RHD 230SL this is on the central side of the cast bracket). If the slot is vertical, the eccentric is 50% of the travel, when the slot is horizontal the eccentric is at either the maximum or minimum stroke/position. Visual estimate of the eccentric shows an eccentric throw of around 1 to 1.5 mm. I am setting up at the vertical position so that I can adjust either way (increase/decrease boost leverage).
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