Brake fade

[Mohammed_A]

Guys please help
having a problem with my brakes, i've got the 333 setup up front on my e30 and standard discs at the rear.Fitted new pads, new discs,changed the booster and brake fluid, bled the system properly.
The problem im having is not the usual brake fade that happens when the discs get hot or the brake fluid boils but mine happens mostly when the pedal is pressed too hard, it starts of fine but if held there starts to go lower and lower.
was on an incline yesterday and when stopped, with foot on brake, the pedal slowly goes lower and lower towards the floorboard.
btw. i have also changed the one-way valve which made a very very slight difference, help please.

[jlevie]

The cause of this will be a leak. Check each caliper and the lines for leaks. If none are found the master cylinder is bad.

[Mohammed_A]

Definitely no leaks. Checked that and also the reservoir stays full with no loss of fluid. So I suppose the master is bad?

[jlevie]

The odds are pretty good that the master cylinder is the culprit if the fluid level isn't dropping. A seal failure in the master cylinder can result in a dropping pedal as the fluid leaks past the seal and back into the reservoir.

[Mohammed_A]

Thank u so much. Btw. Can the master be fixed or must it be replaced?

[dude8383]

The odds are pretty good that the master cylinder is the culprit if the fluid level isn't dropping. A seal failure in the master cylinder can result in a dropping pedal as the fluid leaks past the seal and back into the reservoir.

Yup, this just happened to me a two weeks ago. Replaced the master cylinder and everything was back to normal :)

[jlevie]

Thank u so much. Btw. Can the master be fixed or must it be replaced?

Replaced.

Any time I find it necessary to replace a master cylinder I always check the booster for brake fluid. Brake fluid will destroy the booster fairly quickly. So if the master cylinder has been leaking fluid into the booster I replace it at the same time.