I drive a 1985 190 which has a strange brake problem. When I start the car the brakes work fine for a while (about 10/12 applications) after that the pedal gets stiffer and stiffer and eventually the brakes lock up and the car is stuck. I have checked the callipers and they are all free. I have had the master cylinder rebuilt by a local Mercedes shop. There is obviously a slow build up of pressure in the master cylinder which is not released when the brake pedal is released. The shop did fill the master cylinder reservoir very full, but I can't see any other problem. The vacuum boost seems to be working as the pedal gets softer when the engine is first started. What is causing this build up of pressure?

The master cylinder is at fault. Whether there is a second problem with the system is still a question. DOES the car have ABS?

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

The car does not have ABS. I suspected the Master cylinder, but how can I test it to isolate the problem? Before the shop rebuilt the cylinder I took it apart and cleaned out all the passages etc. Unfortunately I don't have a manual to check that all the seals are in correctly. At the rear of the master cylinder reservoir there is a hose connection which is full of brake fluid. Where does this go and should it be full of fluid? Could overfilling the unit cause any problems?

Check hydralic brake hoses. They will stop up and let fluid go to the calipers but will not let the pressure release

Thanks for the idea about the brake lines, but I can reduce the pressure build up by cracking open the connections to the brake master cylinder - so the problem is not in the lines. If I could buy a new cylinder, I would probably solve the problem, but I can't find one in Egypt. I guess I will have to wait till my next trip back to the USA and hope that there is a cylinder or repair kit that fits my model. I just wish I could figure out why the cylinder is building up pressure without releasing it! Thanks for all your comments.

I have a 85 190e 2.3 and it's doing the same thing.. I just spent $500 to get a new master put on buy a professno and that didn't NOT fix it !!! Now they say it's the booster but they are unsure. Email anthonyjcarrillo83@ yahoo Com

Suspecting booster and or vac lines, but would do the hoses first at this age.

__________________
Prost!

the pushrod needs to be inspected that applies the force to the master cyl piston. the distance it protrudes from the booster needs to be compared to a good working vehicle.
the pedal and the brackets braces etc that carry the pedals need to be inspected for a cause if pushrod is too long.
extra parts being found at the area where the pushrod contacts the piston, or something at the snap ring in the m/c bore between the piston and the snap ring.
i have seen cars where there were also 2 places to attach the booster rod at the pedal. look in all the above areas for something thats making it protrude or not allowing it to return fully.
or/ the cars bent. someone had to slam the brakes on with both feet and stood on it and they were big and strong. now the distortion of the parts is not allowing full return???
or you could stay close to home till you save up for some more parts

+1 on the brake hoses.

__________________
1986 300E (3/2002) Rear ended and totaled (10/2009)
2000 Xterra (5/2000)
1992 400E (11/2009) -Sold
1986 300E (12/2009) - Sold
2004 E500 Wagon (6/2013) - Sold