Locked OSF Brake!

Alluminio · #1 15th January 2015, 09:26 AM

All has been quite well with my A8 D2, which I purchased in November 2013, apart from a couple of niggles, and a new front brake calliper and suspension arms fitted in November 2014.

Yesterday something more dramatic happened; the OSF brake is locked on.

All was working well until I went out in the snow and ice and enjoyed a little bit of spirited driving, and thus the ABS and TCS were in action a few times. All was well until the ABS was activated as I approached a junction and then made left turn to stop at a garage. After restarting to leave the garage, the car felt as though a very effective parking brake was on - which it wasn't.

Lifting off the throttle made the car lose speed very rapidly and as I reached home about 2 miles away, there was smoke from a very, very hot OSF brake.

The NSF brake calliper was replaced as the piston was corroded, but it never fully locked. Up until yesterday there was no sign of sticking on the OSF brake. The car has only a genuine 73K miles on it and until the recent calliper, all the components on it as it left the factory.

I am wondering if the ABS unit may have a jammed valve, given that until yesterday, my usual driving style means I can't ever recall the ABS of TCS having been previously activated!

There are no warning lights illuminated.

I have a fully-blown VCDS to interrogate the car's systems, and whilst I will be doing this later today when the temperature rises, I wonder if anyone else has had this situation?

TIA

Mark

Adrian E · #2 15th January 2015, 09:41 AM

I doubt ABS - if the controller was knackered it'd throw a warning light on the dash and deactivate the system. Sounds like the caliper has cried enough - possibly if it looked OK when the other side was changed, now the pads are worn down some more they may be more corroded looking?

Alluminio · #3 15th January 2015, 10:52 AM

The caliper was my first thought, but as it stuck so suddenly, thoughts of something more complicated arose!

A caliper is an easy, and compared to an ABS pump, relatively inexpensive fix.

Here's hoping...

Any suggestions on the best source for a reman. or new caliper. IIRC the last remanufactured one was c. £130.

David's8 · #4 15th January 2015, 11:56 AM

Stuck piston(s) in caliper is my bet too. Especially when driving in mucky, salty crud - that may have been the last straw.

Delboy · #5 15th January 2015, 12:12 PM

Yeah definitely the piston stuck or caliper not free to float, I had a bike to take for an MOT years back and on the way there it started to slow down, so I gave it more throttle, and more and more then bang front wheel locked solid and I binned the bike.

Boss wasn't happy that I chucked a customers bike down the road but **** happens, that was because of a sticky piston, disc heated up, which makes it expand then it locked solid.

Alluminio · #6 1st February 2015, 01:19 PM

The remanufactured calliper eventually arrived and has been fitted, but the brake still binding, albeit not as badly, but enough to get very hot.

Also, the NSR calliper is sticking, which leads me to think there's something afoot with that split braking circuit.

It frees occassionally after a dose of ABS/TCS activity. There's also an accompanying clunk from the front. After another operation of the ABS/TCS, the brakes appear to stick again.

It's intermittent.

No warning lights shown however.

My thoughts are turning towards a sticking valve in he ABS unit?