Brakes comming on by themselves 190D 85 - PeachParts Mercedes-Benz Forum

D VanWalleghen · #1 08-05-2006, 08:30 AM

Hello,
I have a 1985 190D standard that the brakes come on by them selves. I can put my foot under the brake pedal and release the brakes while driving. I increased the strength of the pedal spring it helped some.
Thanks

psfred · #2 08-05-2006, 09:42 PM

That's a weird one!

May be a bad booster, but the only time i've seen anything like this it was a bad master cylinder failing to release the pressure in the lines after application. The resultant heat from dragging calipers then causes brake application as the fluid expands.

I would flush the fluid with top quality DOT 4 fluid and verify that all the calipers are working properly, especially the rears -- the temptation to put new pads on old worn out rotors will cause the pads to bind when the contact the anti-rattle spring, and they drag and heat up. That should not cause you problem since the plunger in the master cylinder should be above the fill bore, but if something is wrong there, you could be getting improper release.

Check for correct positioning of the brake pedal and correct installation of the master cyilnder, too.

Peter

400E · #3 08-05-2006, 10:07 PM

I have read about something similar happening when rubber brake hoses collapse internally. They can then act as a one-way valve, which may prevent the brakes from releasing when you let off the brake pedal.

Were I you, I would most definitely NOT drive this car until I was absolutely sure I had identified and fixed the problem.

D VanWalleghen · #4 08-05-2006, 10:42 PM

Thanks for the help,
I will bleed the lines and see if that changes anything. Currently I am only driving short hops 2-3 mi where there is plenty of room to pull over.The fact that I can release the brakes via the master slinder makes it look like the prime suspect.
Thanks again

dkveuro · #5 08-05-2006, 11:41 PM

Common problem with an old m/cylinder.
The piston is not retracting far enough to uncover the unloader port....pulling back on the pedal is helping it to do so.

.

t walgamuth · #6 08-06-2006, 10:06 AM

is the spring in the mc broken? or perhaps it is gummy and not releasing for that reason?

tom w

dkveuro · #7 08-06-2006, 10:28 AM

Thinking about this last night before I went to sleep I remembered one a while ago with same problem.

To resolve which part of system was faulty, I released the m/c from the servo booster after applying the brake to the point where we had brake drag with pedal released.

As we undid the two nuts holding the m/c to the booster, the brakes ran free...this time it turned out to be the booster piston assembly.

You might try the same before spending on a m/c.

We could see in the end of the m/c and see the plunger had fully returned to the internal snap ring.

.

t walgamuth · #8 08-06-2006, 02:36 PM

thank you.

that details the failed booster scenario.

could you comment on the failed mc scenario, please?

tom w

dkveuro · #9 08-06-2006, 09:52 PM

Quote:

Originally Posted by t walgamuth

thank you.

that details the failed booster scenario.

could you comment on the failed mc scenario, please?

tom w

There's a small port in the m/c that is uncovered by the first wiper seal on the piston.....if the piston does not return to uncover this unloader port...pressure builds up in the brake system and pads/shoes drag.

On 60's and 70's euro cars/trucks, there was a 'residual pressure valve' at each m/c outlet to maintain about 4 to 6 psi pressure in the system.

These had a habit of failing to bleed off pressure and would make the brake operation unstable when applied at speed.
Modern systems do not have this feature as it was primarily to preload the piston seals in the cylinders to prevent system collapse and keep umbrella seals open against the cylinder walls.

It also reduced m/c stroke to give a better pedal feel.

.