I have just replaced the front callipers on my mk2 8v (1990 pb) GTI with a 280mm G60 set up. While the brake system was open I decided to change the master cylinder and compensation valve at the same time. I used a 22mm Meyle master cylinder and a TRW compensation valve. I have bleed the system using a Sealey pressure bleeder (at 20psi max) and then the two-man method to make sure i get all the air out. No air coming through the lines. Brake pedal is firm, but when i run the engine and press the brakes they go slighly firm and then the pedal slowly sinks further and nevers gets rock solid. I first i thought this was a faultly Meyle master cylinder, but i have put my old master cylinder back on and the same thing is happening. Feels like master cylinder failure, but i can't believe i have goosed both as i've been really careful to bleed the system correctly and cleanly. (only used 10psi in the pressure bleeder when i put the old cylinder back on). Is there any other part of the system that would cause the pedal to sink when using the vacuum servo? (rear callipers, compensation valve?) The car is up on stands all around. There is no fluid leaking from anywhere - I've check the whole system. My next step is to try and isolate the rear callipers from the system using some bleed valves in the compensation valve. And perhaps similar in the master cylinder. I refurbished the G60 callipers with a Big Redd kit, so all nice and new seals. I haven't touched the rear callipers, but they were working okay before. Any advice welcome. Thanks.
If you replaced the rear load sensing valve, you may have air in the rear brakes now. Gets covered here about once a month. Otherwise, possibly air in the master, or a dodgy servo. Is there free play on the rod going into the piston?
Turns out it was a faulty master cylinder. I replaced it with another new one and it bleed fine. Have a good firm pedal now and the G60s are working great. Thanks for the comment Tristan - i'm familar with the issues of getting air out of mk2 brakes so was confident it wasn't that. Great to be out driving in the sun today
Hmmm I replaced the rear caliper [leaking] and bled the brakes but my pedal does the same thing but passed mot
Hmmm I was thinking that I did question it but they said the brakes work to a satisfactory standard.... but was thinking the pedal travels?..
Don’t forget while bleeding rear brakes you need to put a clamp on the load sensing valve so that it’s fully open, as with rear wheels off the ground it will be almost closed and not allow air and brake fluid to pass sufficiently in order to bleed the system after the sensing valve.
