Brake Failure

[Kevkeller]

I finally got my fuel pump working and the engine runs well again.  I took it for a ride down the hill.  By the time I go to the bottom of the hill I lost 1/2 my brakes and the front brakes were smoking.  I looked for any leakage around the wheels and did not see any.  So I figured the master cylinder failed.  I bought a new one and replaced it.  Several questions I have.  I am using a new PRESSURE bleeding system. 

1.  Do I still need to bench bleed the master cylinder before I put it in?  I didn't and it seems real spongy just on the front brakes alone.

2.  With the pressure bleeder I am not getting any fluid out of the rear brake bleed screw.  Is this normal?  I don't think so.  I am using about 4 psi, I went to 5 or 6 and one of the brake reservoir float caps blew off and put fluid all over the place. 

3.  I am wondering if the rear brake pressure regulator has failed closed and is not allowing any fluid to the rear brakes?

Any thoughts?

[graphic66]

My experience was that the brake pressure compensating valve needed a huge amount of pedal pressure to get it to start flowing, then it worked great. I had the same experience with the pressure bleeder, they don't work on the newer master cylinder with the floats.I also couln't get the rear brakes to flow with brake fluid. I just finally ended up pushing the brake pedal really hard and it just went down all of the sudden and everything worked fine. I had taken the pressure compensating valve apart to clean it up. The valve is very simple, just a spring piston. I think it gets in a relaxed position and you need to push the piston back to get the fluid flowing to the rear brakes. I did bench bleed the master cylinder in a vice first, but it may not be needed.

[J. Huber]

Just an aside -- be sure to replace the brake hoses, even if they appear fine. This may be part of original prob? Ignore if your way ahead of me here...

[ja17]

Yes, bad brake hoses most likely were the cause of your problem. The bad hoses probably locked up the front calipers. The moisture in the old brake fluid overheated, boiled and vaporized filling  your brake system with air bubles, resulting in brake failure.

Change all four rubber brake hoses and flush the entire system adding new fluid.

[Kevkeller]

Thanks for the responses gents.  Interesting but I thought the failure of the hoses would have caused the calipers to get reduced pressure not lock them up.  I am thinking of taking apart the brake compensator to see if it is rusted shut.

[stickandrudderman]

I routinely fit braided stainless hoses to my customer's cars as swollen hoses are so common. They look fine from the outside butI've actually had one car's brakes catch fire on road test!

[Kevkeller]

Joe A.

Thanks for the response but could you explain why the brakes would lock up with a bad brake hose?

Thanks,

Kevin

[DaveB]

The hoses are probably restricted inside to the point where they don't allow the brake fluid to return when you release the brake pedal. So the brake pads remain clamped to the rotor.

[ja17]

Hello,

Yes, Dave is correct.  The rubber brake hoses get swollen shut after all these years.  When you press on the pedal you generate high pressure which forces fluid through the swollen hoses.  The fluid is not able to return on its own low pressure so the callipers stay on.

[Kevkeller]

Thanks for the reply.

I think my problem is the brake compensator though.  I cannot get fluid to bleed out of the compensator, no matter what I try.  I am using a pressure bleeder and even a vacuum attached at the bleeder screw at the same time.  I have pushed the peddle as hard as I can and the fluid still doesn't come out.  I even bled it to the input of the compensator.

I took the compensator apart and the shuttle piston moves.

This is driving me crazy.  I want to exhaust all avenues before the I drop $300+ on another compensator.

I think the reason the front brakes were hot was because I was riding the brakes down my long steep hill because I have a little valve oil leakage which fouls the plugs when I use the engine to help slow the car.  Tthe compensator wasn't letting any fluid get the to the rear brakes.

[graphic66]

You really need to push hard on the peddle. Try starting the car and with the power assist on the brakesyou may get the piston to move. Mine took some very hard peddle pressure with the car running and it just shot the peddle to the floor and after that everything has worked perfect.
  Did you completely dissassemble the compensator.  You may want to check it again and pull the piston right out. I believe you will get it to work if the piston moves free and the passages aren't plugged. Mine was gummed right up and the first time I took it apart I didn't pull the piston apart. It was the problem. And it took some really aggresive peddle pressure to get it working.

[Kevkeller]

Believe me I felt like I was going to break the seat I was pushing so hard.  I also ran the car. 

I did pull the piston out and cleaned it.  It seems to move fine.  I blew the passages out with air.  I think it was pretty clean and I lubricated it with fluid prior to reassembling it.  There looks to be a plastic valve at the very end of the piston which maybe isn't working correctly.

I will take it apart again.

[jacovdw]

This may be a bit of a longshot, but would it not be better to try and bench bleed the proportioning valve after reassembly.
Considering that air is much more compressible than brakefluid and that any trapped air pockets in the valve can be the cause for the difficulty with bleeding the rear circuit.

It does not form part of routine maintenance to disassemble the proportioning valve and hence, when bleeding the rear brake circuit, one should not experience such difficulty. Or am I wrong?

Have a look at this webpage: http://weboldtimer.com/index.php?option=com_content&view=article&id=61&Itemid=72&lang=en

Very nice description of the proportioning valve (or brake pressure regulator) with photos of explaining the basic operation.

[Kevkeller]

Thank you VERY much for that link explaining the function of the valve along with the diagram.  From the description it does sound like that the fluid flow is unimpeded up to 40 BAR and then it is reduced above that.  Knowing that the fluid should flow unimpeded at low pressures it seems like the bench bleeding shouldn't be needed.  Nor any hard peddle pressure.

If I understand the diagram correctly I think the problem might be the tiny valve (#11) is not allowing the fluid to flow to the end of the piston and onto the wheels.  Maybe it is gunked up.  Hopefully I can take that part apart and make sure it is working properly.

That is an ingenious little testing device.  Maybe if I get motivated I would build one.

This is a great discussion and I appreciate everyone's input.

Kevin

[dseretakis]

Believe me I felt like I was going to break the seat I was pushing so hard.  I also ran the car. 

I did pull the piston out and cleaned it.  It seems to move fine.  I blew the passages out with air.  I think it was pretty clean and I lubricated it with fluid prior to reassembling it.  There looks to be a plastic valve at the very end of the piston which maybe isn't working correctly.

I will take it apart again.

When your brake hoses are swollen you will have a very hard time bleeding the brakes. If you were to retract the calipers to change the pads or even replace the caliper the pistons won't clamp against the rotor. If you were to drive the car with a non functioning brake you will eventually get some fluid past the swollen hose obstruction and get some braking action. The problem is that the fluid won't return past the swollen hose and the pads will lock up against the rotor causing your brakes to overheat. Basically your swollen hose acts like a one way valve. Your original problem sounds like swollen rubber brake lines.

[Kevkeller]

Hi,

I am quite confident that my problem is the brake compensator.  I have tried pumping fluid though the compensator without the lines running to the brakes attached, taking the hoses out of the loop.  Nothing will come out. 

When I take the piston apart there is no flow inside of the piston to the little valve (#11).  When I get home in 2 weeks I will try again to take the piston apart to find the obstruction.  I might first try soaking the end in some type of fluid that hopefully will clean out what ever is obstructing it.

Having said that, I did order new hoses but I don't think that was ever my problem.  I live up a long steep hill and I believe I was riding just the front brakes because the rear ones were not getting the fluid, because of the compensator.  The front ones could not absorb the energy alone and overheated.

Thanks

[Kevkeller]

OK.... I did find the problem.  It was the innards of the piston that had seized up.  To get the piston apart without damaging it since the hexagonal cap screw did not want to come off, I drilled a hole in a piece of wood and cut the hole on half.  I then put the piston inside the two pieces of wood and clamped it hard in a vise.  After some cranking on a wrench it finally broke free.

There is a tiny little brass piece that the spring seats on.  This piece had gunked up and would not move.  I tried several avenues but ended up taking the rubber seals off and heating the aluminum piston which finally dislodged the little brass piece you see in the picture.  Hard to believe I lost the rear brakes over this tiny piece.  Since I destroyed one of the rubber seals I am waiting for a replacement.

[graphic66]

That's what I had to do to mine. It still took a huge amount of pedal pressure to get it to bleed initialy after cleaning it.

[Benz Dr.]

Graphic 69

You shouldn't have to push any harder than normal. Of course, if you only weigh 85 pounds that's the reason why you can't get any fluid to shoot out the other end.

 

                                Nice work Kev.  You're hired......

  

[Kevkeller]

Ok... got the new seals (from Hungary).  I ordered several knowing I would probably break one or more.  Of course I broke the first one trying to put it on.  I was afraid to try another because this seal has to be stretched an incredible amount so I went to a hydraulic repair center.  They took one look at it and said they had no idea how to stretch it without breaking it either.

So I pulled out my small lathe and made a cone out of some plastic rod.  It was still hard to get it over the cone but it worked like a charm.  I have a couple of the seals left if anyone is interested.

Tomorrow if I have the time I will install it back in the car.  Can't wait to have it working again.

[graphic66]

Graphic 69

You shouldn't have to push any harder than normal. Of course, if you only weigh 85 pounds that's the reason why you can't get any fluid to shoot out the other end.

 I thought it was not right, I wiegh 230lbs I could probably break the pedal off . but I searched some old archives here and another person commented on having to push the pedal real hard to get the compensator to work initially. I had even taken it back out to check it, then when I put it back, same thing. With the car running, I pushed the pedal very hard and the pedal shot right down to the floor. Went and bleed the brakes and they worked perfect ever since. It may be how the piston is positioned or something. But it worked for me. I had taken the compensator apart to clean and check it. It may be if you just take it out of a car and put it back in without disassembly they just work without the hard pedal pressure. I am not sure, almost everything I learn is from a web site here or somewhere else and I have to depend on others experience for most of my knowledge with this car.  I had replaced the master cylinder also from the drum brake MC to the disc brake MC during my axle replacement.
  I just gave them the best test so far a couple of days ago, deer in the road. Thanks to my 100/80 watt Hella H4 headlights I did see them early enough, but at 65MPH I really got on the pedal.  These cars can stop vey well.

[Benz Dr.]

All joking aside, ( ' cause that's what it's been ) you should be able to bleed the rear brakes using normal pedal pressure. No resistance means something's wrong and too much means something else is wrong. In this case air or a restriction of some sort.

I would think the pressure regulator rarely fails but clearly it will once in a while.