Okay something else. if you have traced it down to one caliper. i once had to do this... i had a new caliper, that would not bleed out properly. it bled, no more air bubbles, etc. but i had the exact same issue. i did the crimp test, above, and traced it down to the passesnger side rear. no matter what i did, brakes for crap. almost no pedal, and you better not be traveling fast if you needed to stop.
new pads, new brake booster, new MC. all new fluid.....
anywho... what i finally had to do, (this was told to me, and it sounded stooooooopid, but i figured i would try anything at that point!), was to remove the caliper from the rear disc, (brake line still attached). I put a piece of wood, that i had cut down in place to "simulate" the caliper. i turned the caliper upside down, and had my wife pump up the brakes. low and behold, AIR!!!!!, i used the c clamp to close the caliper back in, put it back on the rear disc, and had perfect brakes to the day i sold it, almost 3 years later.......