All brakes binding, rear lights on

[Goran]

Hi all, I hope you are all well, I have been away for a while.
I drove my 8 to a MOT test today, so exciting to drive it after its been sitting for a few months.
I passed the MOT, and promptly all my brakes failed


As he hands me my keys the MOT guy tells me “your rear brakes are binding a bit, do you live far”
Then he tells his colleague “I need a cup of tea after this one” 😂
Sure enough when I try driving the car out of the industrial estate the brakes are dragging, by the time I make it to the entrance of the estate the car will barely move in 1st gear at 4000 revs.

All 4 discs are hot and the brakes smell. The rear lights are on constantly and the brake pedal is slammed to the floor, rock solid.
I got recovered on a flatbed to my local mechanic. The RAC guy tells me he thinks the mot tester must have stomped on the brakes too hard.

I’m guessing its the master cylinder?

[Goran]

The mechanic is busy, he won't get round to my car for 3 weeks.
Is there a bodge I can do to release the brakes a bit to get the car home?
I was thinking bleeding off a bit of brake fluid?

Tried moving the brake pedal back with foot wedged under it, won't budge.
With the vac hose to brake booster disconnected car moves a bit more freely, but as soon as brake discs warm up it locks up again, even going at 15~20mph.

[MikkiJayne]

Its a weird one this. I've never seen anything like it before.

You could open the bleed valves on the calipers to relieve the pressure, but if the brake pedal is on the floor then its not really driveable. Can you pull the pedal back up? I'm wondering if its the servo which has failed and pulling the pedal down, rather than the master cylinder.

[Goran]

Originally Posted by MikkiJayne

Its a weird one this. I've never seen anything like it before.

You could open the bleed valves on the calipers to relieve the pressure, but if the brake pedal is on the floor then its not really driveable. Can you pull the pedal back up? I'm wondering if its the servo which has failed and pulling the pedal down, rather than the master cylinder.

Hi Mikki!

Yeah its a weird one! I was hoping you would see this post, I was curious if you had hear of this before.
I tried pulling the pedal back up, so did the RAC guy, it wouldn't budge. Even with the vac hose disconnected.
Where is the servo? Is that the big black round thing attached to the pedal box where the vac hose goes?
My understanding of how the brake pedal works is not good lol.

Thanks!

[steamship]

Originally Posted by Goran

With the vac hose to brake booster disconnected car moves a bit more freely, but as soon as brake discs warm up it locks up again, even going at 15~20mph.

Sounds like you may have read this thread (which MJ participated in), as that was their solution:

https://www.audiworld.com/forums/a8-...rners-2912722/

Since it just went through the MOT, that would eliminate air in the system or a leak, which just leaves the brake booster and master cylinder. Do you have access to VCDS (or similar) to check the ABS for errors?

[MikkiJayne]

Crikey. I don't even remember having an account on Audiworld, let alone that thread. I do remember T8 locking its wheels at random - the pedal worked fine on that though. I did briefly wonder about the ABS pump for this, but the pedal behaviour has nothing to do with that.

I think it has to be a mechanical failure in the servo tbh, or something in the pedal cluster. Removing the vacuum removes its power assistance, but the master cylinder is still pressed closed by the servo. If the master cylinder was jammed closed the servo would simply return to normal position - it pushes on the master cylinder piston, but there's no fixed connection between them.

There are some pretty hefty coil springs inside the servo - one of those may have snapped and jammed it shut. Yes, its the black thing attached to the pedal box.

Its the same between RHD and LHD btw. Prestige appear to have a few in stock

[HPsauce]

Just applying my own (poorly-informed) logic to this:
1. Rear lights will be on all the time if the pedal is even slightly down, that's a simple mechanical switch. Doesn't really help with a diagnosis.
2. If the ABS module is a possible suspect (which it could just be) is there a simple way to disable it, by pulling a fuse or connector, so that it's definitely no longer pumping pressure into the pipes? Though presumably the same effect/test could be achieved by putting the gearbox in neutral, take the handbrake off, stopping the engine and seeing if the car could be rolled.
3. Brake pedal solid to the floor would nonetheless point very strongly to a failure inside the servo.

[Goran]

Steamship, I didn’t read any threads on audiworld, just reporting what happened to mine, and thinking off the top of my head how to bodge it, releasing some pressure from calipers, etc. I tried pulling the vac hose on the day it happened, but now thinking back I’m not so sure it did anything. By the time the RAC guy came everything cooled down and he could move the car easily even with the vac hose connected to the servo. When brakes/fluid were hot it literally took 4000K revs in 1st to move at all.

Thanks Mikki! I might as well replace both master cylinder and brake booster to be on the safe side. I was thinking mechanical failure too since pedal won’t move back. I was just curious why its so strongly jammed. If a really strong spring snapped in the servo that would explain it.

Good point HP, rear light is just a switch, I should have taped it closed instead of disconnecting the battery, doh!

[HPsauce]

Originally Posted by Goran

Good point HP, rear light is just a switch, I should have taped it closed instead of disconnecting the battery, doh!

Or disconnected it.
It's an NC (normally closed) dual switch that is opened by the pedal being lifted against it by the return spring.

[Goran]

thanks, disconnecting it did the trick.
I scanned it, no fault codes related to abs or brakes so could be a mechanical failure as Milki said.