Locked OSF Brake!
 

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

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?

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.

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

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.

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?

Well that's a first to hear brakes behaving like that but if you suspect it is indeed applying brakes on the circuit you can prove it by cracking the bleed nipple when the fault is present and see if their is high pressure in the calipers.

Picked-up the car up yesterday from my friendly local mechanic and the car is OK, but who knows why as he could not get the fault to occur.

Today we had a good helping of snow and I took the car out for a try; irrespective of the snow, it still feels as though it is 'dragging' a little...

Anyhoo, on some heavy snow the TCS disappeared - 310 bhp on slippy stuff is fun though - and the ABS and TCS warning lights appeared. Arriving home I switched off the car, restarted and the warning lights we're off and TCS and ABS were operational again.

I suspect my replaced front caliper was OK and the culprit is a sticking valve in the ABS unit or a dodgy master cylinder.

More investigative work yo follow...

Have you ever had any issues with blocked scuttle drains or water in the car? The brake abs ECU is located under the main engine ECU in a D2 in the central scuttle area on the passenger side so it's always possible it's causing issues if it's been dunked in water at all

Interesting.

I did take out the front mats a few weeks ago to install a Dension iPod interface and noticed considerable water staining of the wooden panel in the footwell. This got me to thinking there might gave been some blocked drains in the past.

I will look further...