Brake issue.

[John.]

Think I have a brake problem......6 weeks ago when out for a run I came to a halt in a lay-by, I let the car roll to a halt without my foot on the brake and expected it to roll back a touch....it didn't. I jacked up all 4 corners there and then but the wheels spun free ??
Then yesterday I drove for 10 miles and when braking at a roundabout I let it roll to a halt....seemed to stop itself.
Got back home, all wheels warm and discs HOT.
I'm not all that sure how the system works compared with vacuum servo set-up but my understanding is that instead of the servo it has a valve unit fed by pressure from the PS pump........would it be likely that at any (engine)speed other than idle, the pressure produced from the PS pump is overcoming a spring/valve at the servo(is this what's refered to as brake bomb?) and applying the brakes .
Is there a way of testing this set-up?
Sorry for the long winded story.
Any help would be appreciated.
Thanks for reading,
John.

[Ryss]

The bomb is a pressure accumulator which stores the power steering fluid under pressure so that one still has brakes if the fan belt breaks or the engine stalls etc. It is located low down on the left, near the left front wheel.

Your brake problem is most likely the master cylinder or the brake pedal that is not returning to its normal position. if its not the brake pedal, I would have it checked by a brake specialist.

[johne]

Although there's more than one thing it could be, on our cars the most likely IMO is the seizing of the handbrake mechanism on one or other of the rear wheel calipers.

I suggest you check that the handbrake mechanism is free along with the rear wheels when the handbrake is off and then drive the car without using the handbrake and see how it goes.

[John.]

Will check pedal and rear brake set-up today hopefully.
Car is a 7A coupe by the way.
John.

[andy10v]

Worth giving the brakes a once over. When were pads/discs last changed?

I have bought a couple of cars with the brakes "seized", the carriers where just rusty preventing the pads backing off. Remove the calipers, de scale the caliper carriers (pay attention to where the pads slide). Check the pads slide freely in the carriers, some cheaps pads seem a little big, quick file sorts it. Clean and grease the caliper carrier slide pins, and check they move freely.

Check the handbrake mechanism is free on the rear calipers, and the handbrake cables are ok.

The rear caliper handbrake mechanism is known to seize up, you can normally free it off again temporarily, or rebuild it fully, recon calipers are not too expensive though.

[John.]

Worth giving the brakes a once over. When were pads/discs last changed?

Andy, it had new pads and disc all round last year, the pad carriers were filed flat and good quality(valvoline) copper grease applied, I also stripped both front calipers-new piston fitted to one and all seals replaced, I also de-scaled the calipers where the rust was creeping round to the dust seal and so pushing the seal against the piston(s).
Took the rear wheels off this morning, checked cables-n/s was a little gritty but I freed it off after slackening the cable at the adjuster and unhooking it from the caliper. Anyway...when reassembled all is good, so took for a wee spin of about 12 miles and didn't use the handbrake at all. Got back and all 4 discs hot
Going to get new cables and strip the rear calipers,will check the front are still good too and take it from there.
The joys!
John.

[ChrisD]

John, My money's on the master cylinder. I had exactly the same problem on my ur. The clue is it's all 4 wheels getting hot. You'll probably find that the brake pedal is not returning to the correct position due to the seal failure/displacement in the master cylinder.
This problem is mainly highlighted when the engine has been running for a while.
Replacing the master cylinder is straight forward.
Top tip - go for a genuine Audi part!
Chris

[John.]

John, My money's on the master cylinder. I had exactly the same problem on my ur.

Thanks Chris,
It's good to hear that and what you did to resolve it, I've got a known good 2nd hand one so will fit that.
I haven't been using the car because of this, will get around to it soon.
Many thanks,
John.