Jimbro-
Sucks about the knee. Good to hear you are recovering enough to tackle your brakes again.
Mine are working well enough to tell me chipster probably has the diagnosis nailed, but I suggest replacing the rear cylinders before anything else. You said you tried that, but check the picture from Walt in this other thread and tell us which style rear cylinders you have ( I put it here too). So far, what I call the "evil cylinders" are proving to be a big part of this problem for at least me and another victim (Walt) who figured it out.
http://forums.maxperformanceinc.com/...d.php?t=598515
I had installed new or rebuilt rear cylinders when I first started working on my car. At the time, I had drums in the front. The drum master has a residual pressure valve built into it. The brakes bled fine and worked fine. I started having trouble when I went to discs in the front and replaced the master. Like you, I installed an aftermarket combo valve. When I started having trouble, the rear cylinders were the last thing on my mind. I wasted hours and money replacing everything else.
I ended up adding a residual pressure valve, which was a major pain, so go down that path as a last result. For me, it just confirmed the problem was sucking air through the rear cylinders when the pedal was released. When I am next united with my car, I plan to replace the rear cylinders with the NAPA style for peace of mind.
I was never convinced the aftermarket combo valves have any residual pressure function. I got 5 different opinions from 5 people. I know that mine does not have a check-valve at the rear port (the type valve used in the drum master). The disc master cylinder definitely does not have this function on the rear line. Most people must get away without it. I don't know for a fact, but I think your odds will improve if you use the NAPA style rear cylinders. I do know for a fact it is a cheap and easy thing to try before swapping everything else in the system.
Let us know which style rear cylinders you have....
Left is Walt's NAPA replacement, right is the style I have