Dangerous brake faults!

[mrhappy]

Two worrying problems have recently materialised with my brakes, which would have disastrous consequences if I wasn't now expecting them and I don't get it sorted soon.

 

1)  One two occasions in the last few weeks on long motorway drives I have had no braking at all.  This happens after a fair length of time travelling at motorway speeds, not needing to use the brakes and it's been raining persistently.  The pedal goes right down to the floor with almost no braking effort.   Pedal feel will come back only after pumping the brakes quite hard,  It will deteriorate again if I continue cruising at speed.  Under normal urban and rural driving the brakes are fine.  At idle, the pedal is firm but gradually sinks.  With the engine off, the pedal is rock solid, so I'm certain there isn't a hydraulic fluid leak.

 

2)  When I perform a hard emergency brake, the wheels will lock up and the ABS keeps them locked until the car is brought to a dead stop.  I cannot stop braking and drive out of this situation, the engine fights the brakes but the brakes win, permanently.  There is NO ABS action when this happens - the wheels lock up firm.  There is not sign of ABS or ESP kicking in on the dash - and no faults registered on the car.  This has happened to me once unexpectedly, but I have tested this out safely since and it is repeatable.  

 

Does anyone have any ideas what might cause either of these too malfunctions, or whether they can be related?

 

Many thanks in advance.

[Guest]

You have just opened yourself to legal consequences if you continue to drive the vehicle in this condition and are involved in an accident.

 

There is something seriously wrong with the braking system requiring professional attention.

[J.R.]

Don't know the answer but my unch would be the ABS valve block and/or controller, wheels locking up is a sign of the ABS not working, without it the vehicle is very overbraked or more likely overservoed, its very hard to cadence brake because with the ABS not working they lock up so easily, not releasing though is very scary.

 

It does not sound like you have a fluid leak and were you to have one you would no doubt see the master cylinder level drop but I would point out that your engine off pedal rock solid test can't be relied on because  the servo control loading spring is massive due to the overservoing and you would really need to be bending the seat back to overcome it to test the master cylinder.

[mrhappy]

Thanks for your replies.  @MicMac, I'm well aware of the severity of the potentially consequences.  The issues have only properly come to light today, so I will get it sorted ASAP.  I just like to be informed.

 

@J.R. I had the infamous G201 brake pressure sensor fault a few years ago, and replaced the ABS module with a second hand unit.  I suspect the 'new' module is at fault.  The brakes work fine under normal driving, but the not releasing is very scary indeed.  I've checked for external fluid leaks and can't see any sign, but the effect of the G201 problem is an internal leak in the ABS, as I understand it.

 

[AVHreplica]

I assume you have had it scanned for any possible stored codes?

But AFAIK whatever fault on the braking system would fire up relevants indications..

[mrhappy]

@AVHreplica I've scanned using Carista and there are no related faults.  When getting into a hard lock stop, there is NO ABS light, ESP light or other visual indication of anything happening - it's like the car doesn't know about it

 

Edited 21 November, 2022 by mrhappy

[AVHreplica]

Normally when hard braking and despite wheel locking, the ABS only works (if it tries to unlock wheels) with no any visual indications.

The driver only heads the characteristic sound from the unit trying to keep wheels unlocked.

[mrhappy]

For the benefit of other people:

 

The 'long pedal' (1) was purely a symptom of a badly unevenly worn disk on the inside drivers side.  It was overlooked (by myself) because all other front disk surfaces and pad thicknesses looked fine.  Initial thoughts were that the caliper sliders were seized or binding, but as I replaced the calipers two years ago these turned out to be very free and smooth, so the uneven wear was probably just unfortunate dirt etc. trapped between disk and pad.

 

The dangerous ABS behaviour (2) was down to two brakes lines on the pump unit being incorrectly fitted when it was replaced a few year ago.  The right and left lines were plumbed into the opposite ports. The effect of this is that when the ABS detects one wheel locking it will modulate pressure to the wrong wheel, thus causing an error loop that keeps both wheels locked until speed = 0.  The mechanic/garage that fitted it is no longer trading so this was rectified by another at some expense, which is rather annoying.  It took me this long to notice because I haven't had the need to emergency brake, fortunately. 

 

I have new front brakes and properly behaving ABS now, so all is well.  It's just a shame the clutch slave cylinder has failed on my T5.1 now 😭

 

[Guest]

You will be a real mrhappy now 🙂

[TheClient]

Wow

 Thanks for replying back. That abs connection **** up is a bad one!!