Audi Repair: A4 Quattro 1.8T ABS problems?, chasing ghosts, fluid leaks

Question
QUESTION: Hi. A friend of mine has a Oct' 2002 A4 Quattro 1.8T which just this winter started demonstrating a strange issue with its breaks: When the air temperature was about -10 C, the system would "freeze". The pedal cannot be pressed ("like stepping on a rock") at all. When the temperature is higher, the breaks work, but it seems that some breaking pressure is constantly applied on the rear right wheel. Mechanics originally said that there was water build-up in the break fluids, and replaced them several times, trying to take all the water out. However, the problem re-appeared on the next cold day (which was about a month after the repair job). Now, mechanics are saying that the water build-up is in the ABS system, and proposed replacement (Part No. 8Ε0614517Μ). Do you happen to have seen this problem again, and possibly have an idea of what could be going wrong?

Thanks.

ANSWER: I think the mechanics are chasing ghosts.  I think the problem lies with the master cylinder, if the pedal is as solid as you say.  is there any fluid leaking into the passenger compartment?  Let me know what you find,  Jan

---------- FOLLOW-UP ----------

QUESTION: Hello Jan,
Thank you for your reply. Regarding fluid leaks in the passenger compartment, no, none have been noticed. So you think that it is the master cylinder that gets frozen? Does this also explain the generally erratic behavior of the brakes? Another symptom that was noticed yesterday was that with the gearbox in neutral, on a sloped road, and no pressure applied on the pedal, the car remained stationary. Also, mechanics, at one instance, said they disconnected the ABS, which would supposedly eliminate such issues (i.e. rear wheel constantly braking), but it didn't.

Answer
Sounds like one of your calipers is sticking.  Get the wheels off the gound and make sure they spin freely.  What do you mean by erratic behaviour?  The ABS unit should not contribute anything to the barake system during normal operation.  Disconnecting the ABS will have no effect.  The ABS only gets involved if a wheel locks up under braking.  The master cylinder I would suspect, replacement is messy but fairly straightforward.  Let me know what you find,  Jan