A4, 2.0, 2001 - Brakes remain on after Emergency Stop

[Pickup]

Hi Folks,

After performing an emergency stop (ABS kicks in) the brakes remain on - brake pedal is released.  Brakes release themselves after a period of time (engine off) initially it was 3 minutes then 10 mins now it's 1/2 hour +

When the brakes eventually release there is a clunk from the dash area.

Thinking it may be the servo - anyone come across this fault before?

Does the brake system need a special bleeding procedure (like Golfs for example) if the servo is replaced?

Any help appreciated

Thanks

Faser

[Pickup]

as we say up here - Nae Luck - searched on brakes / seized / siezed / stuck and nothing... well nothing relevant...

Anyway have now got the car - brake pedal does not return as customer had indicated.  Symptons can be replicated - but only with engine running (Servo?).

Brake pedal / lever can be returned to normal position by pulling on brake pedal but brakes still remain stuck - for a few minutes (ie less time than if pedal isn't manually returned).

Checked the brake switch and it appears to be functioning correctly (diagnostics show brake light switch and brake switch actuated and off at appropriate points.)

VW have servo (£220) and master cylinder (£115) in stock - but not the brake light switch (£12) .... shock

Cheers

Fraser

[tim bowkett]

if memory serves i think the servo can fill up with water, and hold the brakes on, i think this is due to where the servo is fixed

[Bonzo]

I'm not so sure about the servo being at fault. If theres a 'clunk' from the dash when the brakes free off, I'd be looking at the ABS unit.

[Angus]

Frazer

Although it could well be the known servo fault; Rob has a good point. 

Are you able to scan the ABS and see if the solenoids are staying locked in live data after releasing the brakes.

[Or pull the ABS main fuse and test it.  N.B. Possibility of a fault code being set.]

[Pickup]

Guys,

thanks for the suggestions, went with the Servo - managed to replicate the fault with the car stationery and engine running but couldn't replicate with engine off, also customer wanted car back for Monday lunchtime.  All back together just need to bleed the brakes. 

Never thought to check the ABS solenoids in live data - but will next time - there was water (and some corrosion) in the servo. 

Will update you all on Monday.

Thanks again

Fraser

[Pickup]

Guys,

many thanks for your help.

Just a quick note to say that the new servo fixed the problem. 

For anyone else reading this post one other possible indicator which I noticed was that when the fault occurrred (brakes stay on after braking hard ie an emergency stop situation) the brake pedal stayed down , if however,  the ignition was then switched off the pedal came up slightly (although the brakes remained seized for anything up to 45 minutes).

Thanks again

Fraser

[Crasher]

This is getting to be a real problem on the A4/A6 and Passat-I have another in now. The servos must draw in a tiny amount of air around the seam but when the drain tubes get blocked they are sitting in water and so they draw in water and corrode up inside. I hate the pedal clip as well, a crap idea. One interesting point is I have yet to do a servo on a petrol engined model but that may be due to there being more diesel powered examples.