OK, as a background, I am having a pedal that sinks to the floor and takes a decent bit of travel to do much at all, but then the brakes do come on quite solidly. I've pressure bled the system many times, and have just put in a rebuilt master cylinder, which I have definitely proved to be good.

Following a brickboarder's advice, I went and started clamping off rubber hoses to calipers. When clamping all 4 of the front caliper hoses, the pedal is significantly stiffer. Clamping the rear hoses does nothing. As my helper took off the vise grips one by one, the pedal sank down a bit further with every one. Now I do remember that when I was bleeding the front brakes before, I could see the shims move and actually see the pistons move a bit. All 4 pisons on each side move fine, so I don't think anything is stuck. So, my guess now is that something is making the pistons sink back into their bores when I take off the pedal. That would explain the low pedal but yet good braking when it does catch. Could the teflon shims be doing this? Are there any anti-flowback valves in the junction block that could cause this? I really think that this is the problem somehow, but I'm not sure at all how to fix it. Any help, as always, is extremely appreciated. thanks,
Nate Gundy
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'86 240DL sedan, 260K miles, M46, K cam, 25/21mm sways, 260 front and wagon rear springs; http://valvespringcompressor.weblogs.us/