RS2 Brake Woes - ABS or bomb?

[afen]

A while back I started noticing something weird while braking hard. The pedal feels a bit spongy and the travel is very long, all the way until it "stops". If i push until it stops and then push a bit harder i can get the front wheels to lock up and the ABS to somewhat engage. By somewhat I mean that the front right wheel locks up but the front left doesn't, and I can hear the ABS pump doing something.

With the engine running I tried pumping the brake pedal, after about five pumps the brake warning light comes on, and disappears after five seconds. This probably means that the bomb is gone, could this cause the brakes to behave this badly?

I have bled the brakes, and I've also checked the brake servo return line, it's fine. There's enough brake fluid and PAS fluid in the reservoirs.

[domas]

does not really sound like abs problem. even if there is something wrong with abs, it is not the only problem.

Re: bomb
does the pedal become hard when you shut off the engine instantly? or you can still pump it once or twice?

Else typical bakre things, like brake fluid (when was it last changed), air in the system?

[afen]

The ABS is not that big of an issue right now, I can make do without it.

I went out and started the car now. After I shut it off I could pump the pedal a couple of times before it got hard.

I have bled all four corners, and I also purged a good portion of the fluid. I do not think it has something to do with the fluid since the braking is the same whether the car is cold or warm.

[domas]

I went out and started the car now. After I shut it off I could pump the pedal a couple of times before it got hard.

This means that the bomb is going bad, but many people have those bombs that can only pump a couple of times and that should not be a problem.

I have bled all four corners, and I also purged a good portion of the fluid. I do not think it has something to do with the fluid since the braking is the same whether the car is cold or warm.

which wheels are resistant to brake? sticky caliper maybe?

[afen]

This means that the bomb is going bad, but many people have those bombs that can only pump a couple of times and that should not be a problem.

which wheels are resistant to brake? sticky caliper maybe?

As the system is bled and the brake servo is ok, the most likely culprit must be the bomb?

I can't say that a wheel is resistant to brake, with the ABS disabled both front wheels lock up evenly. The brakes are pretty good, the problem I have is that I have to press the pedal pretty hard and far to get the brakes to lock up.

I've also checked the pads, and they are good.

[domas]

disable abs to find out.

[afen]

As I said, I pulled the ABS fuse and the brake pedal is the same.

[domas]

I have missed that.

But my question is, after disabling abs, is there a wheel that has particularly different braking force then all the others?

[afen]

No, it felt pretty good without the ABS

[domas]

need to confirm this, try again. but the pedal was still spongy? mixed results.

[afen]

With ABS: Spongy pedal, long travel. Right front wheel locks up when braking very hard.
Without ABS: Spongy pedal, long travel. All wheels lock up when braking very hard.

[domas]

IMHO brakes are not bled properly, most likely in RF wheel.
My advice is back to square one, back to basics. Something very basic, more basic than brake servo or abs is not doing the job properly. Isn't the RF brake support getting sticky?

But take my advice with a grain of salt, as my knowledge is limited. Lets wait for somebody with more understanding to chime in.

[schmidty]

This may well be of no help but I just resolved a similar problem on my C5 A6 4.2. Took the car in for state inspection (like MOT) and inspector reported that the right front caliper was locked up. Since I live in the "Rust Belt" and had to replace my rear calipers a few months before, I jumped to the conclusion and replaced both front calipers with rebuilt units. Power bled the brakes and still had a locked right front caliper. After searching numerous forums, I conducted tests of the vacuum booster and master cylinder. Booster tested fine but even though the brakes felt normal in service, if I had the car in "Park" with the engine running and stood on the brake pedal, it would slowly sink to the floor. Replaced the master cylinder (PITA), power bled the brakes again, and the problem was solved. Neither I nor the tech comprehend how a master cylinder could lock only one wheel (I had put the car on stands and was able to freely rotate the other three wheels). During all this, I had concerns about the servo, ABS, ESP, etc., but I would agree with domas - confirm the basics before tearing into more components.

[popdemonic]

you said you're pumping the pedal 5 times then the warning light flashing.i think the problem is something about insufficient reserve in the system.it's most likely the bomb or bad brake servo.have you tried to pump the pedal with the engine off and see how many times you get before the pedal became rock hard?if you count it for a few times,it's something about reserved pressure assisted problem.flushing the new ps oil,cleaning all oil screen lines are the basic solutions if problem is still exist, i would start to look for bomb or brake servo replacement.

[afen]

I tried pumping the pedal with the engine off, it took about 14-15 pumps before the pedal was solid.

I have checked the brake servo by removing the return line, it is OK.

Should i try bleeding the brakes again? I purged probably 0,5L of brake fluid in total, in this order: rear left, rear right, front right, front left. There was a little air in the rear brakes, and some in the front left caliper.

Is there a possibility of this being caused by air in the hydraulic system?