My son's VRS has a brake pedal that sinks (slowly) down if you continually press it (engine on). It's rock solid engine off and has no fluid leaks at all. Overall the brakes feel a little soft overall but the car goes into ABS etc if you 'exercise' the brakes. 

 

Is this 'typical' for the breed and to do with the diesel vac pump creating a lot of servo effect - or more likely a fault?

 

(It's literally just passed its MOT this week with nothing about this flagged)

 

Any advice gratefully received! 

 

54 minutes ago, Grafix said:

My son's VRS has a brake pedal that sinks (slowly) down if you continually press it (engine on). It's rock solid engine off and has no fluid leaks at all. Overall the brakes feel a little soft overall but the car goes into ABS etc if you 'exercise' the brakes. 

 

Is this 'typical' for the breed and to do with the diesel vac pump creating a lot of servo effect - or more likely a fault?

 

(It's literally just passed its MOT this week with nothing about this flagged)

 

Any advice gratefully received! 

 

Well, it reminds me of a story my dad told about an old car of his with a hydraulic clutch, that would move off if you held the clutch down long enough. That traced to an internal leak in the master cylinder...

They are all like it, most dont realise as they have never tried, its a function of the ABS & stability systems, I have a VAG self study guide on the subject, very technical and complicated but fascinating, at one stage while reading it the penny dropped as to why our vehicles have this particularity.

 

I agree with the test showing an internal leak, it used to be part of the MOT test but I think there is something in the testers manual regarding this on later generation ABS systems.

 

editted, - to prove my theory you could disable the ABS by a fuse removal but you will need VCDS or similar to remove the fault code, beware until this is done the ABS will not work even when power is restored, I found this out luckily by chance recently, low battery had thrown up an ABS fault, the MIL light remained on so I tried an emergency stop, no ABS 😞 and the system without it is terribly overservoed, only a ballerina could cadence brake.

 

I will give it a try next time I have VCDS plugged in.

Edited 19 February, 20206 yr by J.R.

My car has a similar symptom, it slightly "gives way" if you have the pedal pressed and you turn the A/C off, I think it is weird but as the brakes work fine overall I haven't been too worried about it. 

Thanks JR for your help and insight. I'll obviously watch it closely but that gives me more confidence that it's a trait and not a fault. On the road as said the brakes work 'fine' I'll refresh the fluid soon and hopefully that will also sharpen them up a tad as I don't know when the previous owner last changed the fluid or bled the system.