If your master brake cylinder is broken COMPLETELY, the weekend trip may end at the next lamp post, the vehicle in front or in the river bed – around 80 meters below the last hairpin bend.
However, this only happens quite rarely – usually the brake pedal becomes “somehow soft” over months or years and loses its pressure point. Electric drivers are often even less aware of a leaking brake master cylinder because the recuperation elegantly distorts the feedback from the footwell.
As a rule, the central organ of the brake system does not leak to the outside, but to the inside. Finally, two pistons work in its cast housing, pushing the lifeblood of the brake system to the ABS unit and the wheel brake cylinders. It does not matter whether it is a tandem cylinder with a snifting hole, central valve or plunger piston. In principle, two pistons with sealing sleeves always slide back and forth in a cylinder. And if the seal no longer seals, the thing can build up pressure but cannot hold it in the long term.
To diagnose such a pressure drop, you could simply clamp a pressure gauge to a bleeder valve (actually a good idea, maybe we’ll do an article on this sometime), pressurize the system and then wait and see what happens.
In times of cheap diagnostics, however, you don’t even need dirty fingers: A glance at the measured value of the brake pressure sensor from the ABS block is enough. After all, every current anti-lock braking system has at least one pressure sensor installed, which can be read out via VCDS, for example.
VW prescribes setting the brake pressure to 50 bar for many models and checking how far it has dropped after 5 minutes. According to this specification, there must be at least 45 bar left on the system – if there is less, Volkswagen classifies the brake master cylinder as broken.
So a test like this only helps you make a decision if the thing is really broken. And if the car is to roll for a few more years. What about just-on-the-edge brake master cylinders in a consumer car with 3 months remaining on the MOT? It should be borne in mind that the maximum pressure under full braking can easily exceed 200 bar. An in thias case you need the person in front, a lamp post or a solid rock in the river bed.
