[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?