'94, stock brakes, w/ ABS, 929 MC, car has been sitting for 4 years. ABS pump was emptied out (simple pouring) and reinstalled.

I go to bleed the brakes and the RR bleeds fine, then I go to the LR and nothing is coming out. After several attempts with traditional pump bleeding, I can get a tiny bit to burble out. If I remove the bleeder completely and press on the brake, only a bit will dribble out (should shoot out). If I hook up a power bleeder, I can only still get that tiny bit to dribble out, like it's blowing bubbles in the tube. If I unhook the brake line from the caliper, it comes out normally from the line. If I pull the e-brake, the caliper clamps down normally, and lets go when I release it. I continued on, and the FL and FR bled normally.

I did a "lazy" rebuild of the caliper (replaced the piston seal and dust boot, cleaned out the cylinder), and still get the same results. During the rebuild, I couldn't find a single thing blocking the passage between the brake line inlet and the bleeder outlet. By theory of isolation (if the RR bleeds fine, it must not be anything in the engine bay, if the flexible brake line leaks fine, it must be after that brake line, and if it's no better without the bleeder screw, then it must be before that...), it looks like the caliper is the culprit and I'll have to replace it, but I am absolutely stumped as to why, since I could see a clear path between the inlet and outlet of that cylinder. Even if the piston was siezed, the power bleeder should be able to move fluid through.


Any ideas?

 

did you bench bleed the master? but im going through the same thing...anyone wanna chime in. i know that some one has an idea.

 

Maybe you aren't truly cracking the bleeder screw open enough?? Ionno man this is weird lol.

 

Yes, I did. At this point I'm going to drive the car and give it some solid braking and hopefully that will shake something loose.
Definitely was, and removed it to make sure that wasn't the problem.

 

If you have the time and tools try remove the line from the master an from the caliper and blow air through and see if pressure is there Just an idea

 

Sorry if you have abs remove line from abs and blow to rear

 

If I understand you correctly, you've isolated the problem to within the caliper assembly, so:
- bleed screw clogged?
- maybe you missed something on the caliper rebuild?

If you eliminate the screw, then maybe something is blocking flow where you can't see it in the caliper (I haven't rebuilt a rear caliper, so I'm just guessing here), and perhaps reverse bleeding the caliper itself (disconnected from the line though) would dislodge it (sort of a Heimlich for your caliper, so to speak).

 

I could try that, I guess blowing through the caliper backwards. Still, though, I was able to go through and see through the passage without issue when I had the caliper empty.

 

The problem is occurring for both rear brakes now. Oddly enough, the RR bled fine the first time, then the LR wouldn't bleed properly, and since then, both have been bad.

I've replaced the brake proportioning valve (albeit with another used, but working one), and still no results.

I did a 929 MC upgrade. There are two lines coming out of the master to connect to the firewall brake lines, does it matter which one goes to which line?