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?