Hi all, I have a 2005 B6 S4 6MT. About 15k miles ago I did a full engine replacement due to cylinder wall scoring, and at that time did basically every maintenance and reliability update I could including a new stock Luk flywheel and SAC clutch setup, as well as the USP slave cylinder and stainless pressure line. Everything went fine with that install and I had no issues gravity bleeding the system.
Fast forward to recently; I was driving in a normal manner (not launching etc) when I noticed a third gear upshift felt a little difficult. Then, I started hearing a weird buzzing/groaning sound coming from under the dash. I reached a stoplight and could not get the car into any gear; I finally managed to jam it into third and limp it home. On every shift that same sound was coming from under the dash after lifting the clutch pedal. Once home, I shut it off and immediately could shift into every gear with ease, ruling out a linkage or alignment issue. At this point it seemed clear the issue was the clutch master cylinder; the car has 130k miles and it’s the original one, so I wasn’t that surprised. To confirm the diagnosis, I found no fluid leaking anywhere from the lines or out of the bell housing, nor was the fluid in the reservoir low or going down, and I was also able to pump the clutch up to get it to go into gear so that I could move it around my garage.
I replaced the master cylinder (after receiving a defective one first and installing it only for it to leak into the cabin) with a stock Sachs replacement and that’s where all the issues have begun. I cannot get this system to bleed. I have gravity bled, I have buddy bled with pedal pumping (both with the bleeder open and single press down, then close bleeder, and also with the bleeder closed, pump up pedal, crack bleeder and close again). I have pressure bled multiple times with a Motive pressure bleeder; Bentley manual says 2.5 bar (36psi); the gauge on the Motive goes to 30, and I’ve done it at 30, 25, 20, 15, etc. I’ve reverse bled it by pumping fluid out of the nearest brake bleeder using the brake pedal and into the slave bleeder as a “closed loop” approach that some folks reported success with on VW’s. I’ve probably put about a gallon of brake fluid (Valvoline DOT 4) through this system. Sometimes a bubble will come out and often a fizz will appear in the tube when pressure bleeding which I believe is coming from the bleeder threads since it stops when I wiggle it around. Throughout all this, there are zero leaks (aside from various fluid spills at the reservoir which is always fun), and the pedal feel is basically the same aside from occasional times that I probably have drawn in air for one reason or another and it gets soft and then firms up with some pumps.
My buddy helping me is more experienced than me and is completely dumbfounded. He sat there pumping the clutch for a while one of the days we worked on it and watched bubbles slowly coming up out of the reservoir every once in a while as he was continuously pumping, but still, every time I start the car up, it will not go into gear. And still, pumping up the clutch a few times builds enough pressure to get it to go into gear. It’s on 4 jack stands right now, rear slightly higher than front, so the wheels are freely dangling in the air. Just tonight after another pressure bleeding attempt I was able to pump the clutch to get it into gear and then held the pedal down for like 3 minutes straight and the wheels did not begin turning, so it doesn’t seem to be decaying pressure off, but that refill stroke just pulls in air somewhere, or something. If I don’t pump the clutch enough times, and I don’t have my foot on the brake, you can see the wheels start spinning once I try to push it into gear, as the synchros are able to operate with no resistance on the wheels.
I’ve replaced the feed tube between the reservoir and master as it was looking pretty chewed up. That didn’t help, but now I feel better about it. I’ve now ordered a new USP slave cylinder and stainless line just to replace them even though they’re not very old at all in hopes that it will be the magic bullet, a bypassing slave that isn’t leaking externally, but I’m getting pretty frustrated. I’m not one to throw in the towel and take it to a shop, but furthermore if I do limp it there, I don’t know what they would do that I haven’t. The only thing I haven’t tried is vacuum bleeding; I don’t have a setup for it but I would buy one if it would work, but I feel that air would just get sucked in through the bleeder threads during that procedure. And of course, through all this, the access to the bleeder is atrocious and I can barely get my hand up there to open and close it, so doing much with it without lowering the rear of the trans seems unlikely.
I don’t believe this is a mechanical issue (separated friction disc, SAC ratcheted down, broken clutch fork) due to the fact that it held power just fine when I was driving it during the incident and I am able to get it to disengage with clutch pumping and it stays disengaged until I lift the pedal.
Does anyone have any ideas?