[Jimbro727]

Originally Posted by Shiny

Guess this means you have not figured it out?

Me either.

Interesting you have the combo valve up at the booster. Me too, even though mine's a LeMans... I don't think it matters, since lots of aftermarket brackets move the valve up top.

Your comment about the bubbles at the master and air rising in the lines even under pressure is also something I chased. Does your master have bleed screws? My new one does, and I noticed I was sucking air back into the master when I bled using a "one-man" bleeder bottle, so I sealed the bleed screw threads and it eliminated that symptom. After that, I have not seen any more bubbles from those bleeders, so I don't think air is trapped between the valve and the master on mine. My test with the master ports plugged supports my conclusion that air in the master is not my problem, even though I keep getting told that.

My last attempt at bleeding, I saw no bubbles at master or any wheel, and I got a firm pedal with engine not running (no booster). So I got cocky and drove around the block. Within about 5 braking attempts, the pedal dropped back to near the floor before stopping the car.

I now believe the fault may well be the master cylinder not returning as mentioned in the post below. I have not looked at mine again, as I now have a much bigger problem to deal with... but I plan to loosen the adjuster between my pedal and the booster and see if it helps. This is my last hope for a fix.

It might make sense if, as described by ericwy/sts, the "refill" port is being blocked by the piston not fully returning, then it might effectively pull a vacuum on the rear system and either pull in air or more probably, pull the rear cylinders away from the shoes. If this happened, I would think it might act like the rear shoes were adjusted way too loose. If this is true, then the pressure build-up in the rear system would be way late, delaying pressure build in front. Just speculating, but it makes some sense... and I need an explanation that makes sense to keep from going crazy.

Along the same line, is there any chance the booster is "boosting" without me pressing the pedal? If so, could this be another explanation for why the master is not returning correctly?

Sorry for the delay - I wanted to work on this a bit more before responding.

My master doesn't have bleed screws, I've never seen one like that - where did you get it?

I don't believe that my problem is the m/c not fully returning, because I can feel in the pedal when the booster is engaging the M/C. I suppose this could be something else that I'm feeling...

I just spent the past few days bleeding. First I bench bled my master again, and got some air out. I then hooked up my pressure bleeder, and left it connected for a few days. Two days last week and once again today, I spent an hour or so and bled each corner. I took around 40oz or more out of each corner each day. The first day I got bubbles everywhere. Second day I think I got some bubbles out of one of the rear brakes and nothing out of the front. Today I got no bubbles out of the rear, nothing out of the passenger side front, and a ton of bubbles out of the driver's side front. I waited until I thought there were no more bubbles up there, then decided to test it. I've literally made no progress. The pedal feels firm without boost, and goes right to the floor with little resistance with boost.

I did notice something interesting, though. With the M/C cover off and the engine off (no boost), the M/C will shoot fluid out of the reservoir with a light tap on the pedal. With the engine running (boost), that same tap yields no shot of fluid. In fact, mashing on the pedal doesn't even shoot fluid out.

My pressure readings at the driver's side front caliper show that I still get virtually no pressure until very very late in the pedal's travel (almost on the floor). Without boost, I get around 100PSI with the pedal as far as I can push it (I believe it's as far as it goes). With boost, I get around 200-225PSI with the pedal on the floor if I press it quickly.

It is possible that more air needs to settle, so I'll continue bleeding. But my gut tells me that there's something else wrong here. People drive around with air in their lines, I think I'd only see something like this if my lines were FULL of air.

Any thoughts?

As to your question about the booster boosting without you pressing the pedal, I don't think that's possible. I've looked over diagrams of how these boosters work, and I don't see how it could do that. I'm by no means an expert, however, so take that with a grain of salt.