Fading brakes- 92 W250

[RDBuck]

I've read all the posts on this issue. I cannot find a definite answer although the problem seems to be fairly common. Entire brake system has been replaced except for the booster. New rotors,calipers, lines, hoses, wheel cyl, master cyl, drums,pads and shoes. [ $1700] Still have a pedal that is firm ( not hard) at first but with steady pressure will quickly fall away until no pressure. Lines have been bled many times both pressure and with pedal. Master cyl. has been plugged and is firm as a rock. The "ABS" unit on rear frame has been bypassed. At this point my suspicion is the proportioning valve next to the master cyl . Something is allowing the fluid to "recirculate" without creating a leak and this is the only thing left in the system that could do that. Two questions.



Has anyone found this to be the case.



It is a dealer item and discontinued. Where do I get one?



Thanks for any help



Richard Buck

 

[B.G. Smith]

Although there are two lines from the master cylinder to the proportioning valve, both lines should have the same pressure so the fluid cant re-circulate unless the valve is by-passing fluid inside and either the front or rear section of the master cylinder is leaking by the piston. I would say it has to be the master cylinder. bg

 

[RDBuck]

Soft pedal

Thanks for the reply B. G. I agree the symptons would lead to a faulty master cyl. I am 99. 9% sure it is not as we have put 3 MC on- the last one from another vehicle with no such problem. Plus the brake line holes were plugged as a further test. I want to replace the proportioning valve as it is the only thing in the system still original. The truck has done this before but seemed to solve itself almost as suddenly as the problem first started. This time I would like to find the source of problem.

 

[cerberusiam]

Yep, same problem on mine. Has to be the ABS unit and its the usual problem.