Rear Brake Delay

[sem9999]

Hi,

Car just been taken to MOT and failed on something never heard seen before by tester.

Wonder if anyone could make suggestion:


Rear bakes engage and release (equally on both sides) 2 secs behind front (which work as they should).


That is when brake pedal is pressed, front come on, rear come on 2sec later. When brake pedal is released, front release and rear release 2sec later!

Any ideas what could be causing this behaviour?
Thanks

Car is a 2.5TD VSX M reg.

[DerekW]

Just a wild guess and I expect to be told I'm wrong, but it sounds like a sticking piston in the brake control valve.

Derek

[jorgy9]

....or air in the rear brakes, typical, just bleed them. Haven't you noticed your tail lifting up while braking to a stop?? It shouldn't lift at all in a bled XM, it brakes flat (front only dives).

BTW, I was reading that many or all the hydraulic Citroens *are* designed for the front brakes to come in first. This can be seen in the brake valve's design of the car. There's one piston for the front brakes, that is pushed first when u depress brake pedal, then this is pushing another piston (in line) for the rears, through a spring. That's why rears come in later. It depends on brake valve the car is equipped with, there are designs with 2 and with 3 pistons afaik (don't know the diff.). Not sure what the XM has.

regards
George

[DerekW]

"There's one piston for the front brakes, that is pushed first when u depress brake pedal, then this is pushing another piston (in line) for the rears, through a spring. That's why rears come in later."

Yes, but not two seconds later, and certainly not releasing two seconds after the front ones. That's why I suggested a sticking piston in the brake control valve, the piston that applies and releases the rear brakes.

So there you are, sem9999, bleed the rear brakes and if that doesn't work, strip the brake control valve - unless anyone else has a better suggestion. You'll find it under the scuttle on the left hand side.

Derek

[sem9999]

Bleeding rear brakes did the job, and its now passed its MOT.
Thank your for all your advice, could not do without all your dedicated help.

[jorgy9]