Brake pedal holds Pressure when engine is off...loses pressure when turned on - PeachParts Mercedes-Benz Forum

Diseasel300 · #5 08-09-2017, 10:50 AM

Quote:

Originally Posted by optimusprime

Press the pedal down on the floor with out the engine on . Now start the engine still holding your foot on the pedal , it should lift you foot up off the floor to normal hight if booster is good.

WRONG!!!!!!!

The vacuum assist is to help PUSH the pedal down, not pull it back up.

Without the vacuum assist, it is hard to get sufficient brake pressure to diagnose a master cylinder unless it is extremely worn. When the vacuum builds to operate the booster, it's MUCH easier to push the pedal down. Since you have more force available to push the pedal, smaller leaks and faults will show up.

A sinking pedal is a sign of a leak or a failing master cylinder. If you find no leaks anywhere (check caliper seals, hoses, ABS pump, etc) suspect the master cylinder.