Breaking Badly... ABS?

[SAJanesick - Bellsouth]

Among other cars, I have two 200TQ20V's - both currently disabled enough to be off the road.  Now that the great freeze of 2014 is over (we only got about 8 inches, but it was the biggest snowfall around here in over a decade) and spring weather arrived this weekend, I decided it was time to get some cars back on the road.  Although embarrassed not to have at least one of the Ancient Audis on the road, we were still served by our '03 A6 Quattro, so shed no tears for me - at least not yet.

After over three year of stasis, I started up the high mileage 200 with relative ease.  At first I was cursing as the engine turned and turned but never caught.  I checked connections and plugs, kept charging the battery and drained my 27 year old can starter fluid.  Eventually it occurred that I should check the fuse box as maybe, just maybe I had unplugged the "Motor" paired fuses to reduce any battery load from the ECU.  Sure 'nuf, the fuses were resting inside and unseated.  With that addresses, I moved the car into the garage for overdue brake work.

Just before the car came off the road, the right front brake caliper had begun to randomly stick.  At first it was just an annoyance and I wouldn't even notice it until on a very slight grade which normally would have allowed the car to roll unless brake was applied.  It was just a very small drag that would come and go over time.  However, it eventually increased in both frequency and intensity and it became the primary reason why I took the car off the road.  I kept saying "I'll get to that next week" for something in excess of 150 weeks.

Friends and local mechanics (often one in the same) rendered multiple opinions, the first and foremost was to replace the brake line which was original save the fact that the previous owner had taken Audi up on the G60 "upgrade" to address the UFO warped rotor problem.  Indeed the G60's are smooth, but inadequate.  Regardless, new brake lines made no difference.  

The next move was to replace the suspect rotor.  Once the system was properly bled with pressurized bleeder system on loan from another mechanic/friend, I had great pedal feel and a very modest test drive showed that all four wheels were getting good braking as the rotors went from the all too familiar rust brown to a shiny polished steel appearance.  

However, some good things just keep getting better and better.  The brakes did improve through the test run.  ABS seemed to be effective under very hard stops and even pedal feel seemed vastly improved over the short 5 minute test span.  But then when coasting up to a light, I found that I could stop, without using any brake pedal at all.  Back at the garage the right front wheel was much warmer than the others and could not be spun by hand.  Hmmm.  Not the caliper.

I opened the hood to look at things and while walking around the car taking the temp of the other wheels, something new occurred.  Just as I was approaching the left front wheel from the drivers side door, I heard a little noise from under engine compartment.  I sounded a bit like a sigh, although it also has a distinctive hydraulic quality to it.  Immediately, checked the right front wheel, and although it was not completely free, it was again spinable by hand.

After referencing The Bentley, reading up on SJM website and poking around on the forums, I'm left with questions as to the next repair step.  Initially I was focused on the ABS Hydraulic Modulator as it has separate output brake lines to the right front and a "return flow pump relay".   It could be something in the relay, or the return pump, or the line to the right front - or given the Audi Gods, it could be all three.  Additional exploration made reference to a sticking master cylinder return port that could be blocked and that can keep the calipers from releasing.  Seem reasonable in that the "release sigh" could easily have come from the master cylinder, although the ABS modulator is also in the same area.  Perhaps the critical difference is that just the right front is affected and not the left front.

At this point I lean toward replacing the master cylinder, after all, it is the original unit which is now 23 years old and has almost 240k miles on it.  Nothing lasts forever, plus it's a whole lot less expensive and has fewer connections than the modulator.   Any other insight on the probable cause and possible solution will be appreciated.

I've only had one other ABS fault experience and that was with our 1994 Volvo 850.  It was also a problem with the right front brake, only in that case the caliper was receiving no hydraulic pressure at all.  When finally diagnosed, I purchased and installed a used unit which immediately fixed the problem.  I always thought it strange that Volvo designed a system such that when it failed, it resulted in NO brake to the wheel versus some type of limp home mode that gave you brake, but no ABS.  Seem less than safe to me as I suspect there are any number of 850 owners out there driving around with a faulty modulator which would not make itself known until a situation where extreme braking was required.  Just at the time you need it most, it wouldn't be there.

Again, any thoughts and ideas would be appreciated.

Later...

            - Steve Janesick