03 Audi A4 1.8t FWD brake lock up

There's been a 2003 Audi A4 with a1.8 turbo, front wheel drive at the shop for a week now.
One of the guys drove it and said that after coming off the highway and stopping at a light, the front brakes locked up and the car wouldn't move.
After an hour or more cool down, the car drives normal. We don't work a ton of Euro imports, and the boss asked me to check it out before he let the guys unload the parts shotgun on it.

It had all the abs related lamps illuminated on the dash, the only code was for brake pressure sensor out of range.
I cleared the codes and drove it for HOURS. When a few laps up the highway didn't recreate the symptoms, gave it several panic stops with full abs activation.
With the car on the lift, all the wheels moved freely by hand. I took it home e for the night, back to work the next day, and told him I couldn't get it to act up.
I saw it back today for the same complaint. I didn't get to look at it again myself, but I know they've changed all the rubber lines, the master, the booster, the abs unit and it's still the same.
I checked tsbs, and found none brake related. Anyone know of a similar pattern failure with these cars? It's out of m my hands now, but I'd like to pass down some helpful advice.

I've seen this kind of thing on GM's many times before, but never heard of it on a euro make. I'm figuring the brake pressure sensor is integral to the ABS module?

My first thought was a failing booster, sticking applied and blocking off the return ports in the master cylinder. Then, you said the booster had been replaced

If it ever does decide to act up again, you could always try loosening the master cylinder nuts that hold it to the booster. If the brakes release, then you know the problem is at the booster.

Dependent on accessibility, of course. I think they bury the master/booster in these models...

Tyler wrote: I'm figuring the brake pressure sensor is integral to the ABS module?

My first thought was a failing booster, sticking applied and blocking off the return ports in the master cylinder. Then, you said the booster had been replaced

If it ever does decide to act up again, you could always try loosening the master cylinder nuts that hold it to the booster. If the brakes release, then you know the problem is at the booster.

Dependent on accessibility, of course. I think they bury the master/booster in these models...

You are correct sir. Actually, the pressure sensor LOOKS like it could be unthreaded from the unit and changed, but the code alone wasn't enough evidence for me to go attack the thing.

I saw them pull the car out of the garage, so they must have figured out something. I've just been too overwhelmed to check in with the mechanic who was working on it. I'll get his version and post up on Monday.
I know lack of info makes for poor help requests, especially since I didn't get to see this one all the way through. Thought it might help to get the ball rolling though so to speak.

Ah don't worry about the lack of info! I used to go to other techs all the time with the weird/difficult ones, mostly just asking them to listen and give feedback. Really helped me get the diagnosis moving again.

And yeah, I suspect the code is a symptom and not the problem. Maybe a rationality issue between the brake pedal switch and the pressure sensor?

I could see an overadjusted brake booster pushrod doing this, but the symptom would be more consistent...

Update: I caught up with the guy who ended up fixing the Audi today. He said that he put another used master cylinder in it after he found no fluid coming directly from the master on one leg of the system .
He told me that one line off the master is the feed for one front, and the opposite rear (LF-RR, or RF-LR), and the other line feeds the opposite pair. I thought they generally fed the front pair and the rear pair. I Probably wouldn't have caught that without having read a procedure.

Wha, seriously? So only half the master cylinder was creating pressure?

I'm thinking that only came about when he installed the first used master cylinder and tried to bleed it. I'm not thinking that explains both front brakes locking up. I was never able to recreate that symptom however...

I'm familiar with the design but like Noah says still wondering why both front brakes were locking up.... :blink:

Right, so the first master was a failure on its own, different from the root problem. Always fun when you have to fight parts during repairs!