Sorry for hashing up an old thread...
I got same symptoms, sticky back brake peddle, bled the rear brake, worked fine for about a month, sticky again, removed rear master cylinder, cleaned out some debris inside, seals look great and work good, put back together and nothing, yes, Nothing....
Have plenty of fluid in reservoir, which goes to master cylinder, but have no fluid at brake caliper even with 20 lb pressure on mitivac (even left it on 22lb pressure all night) bled the master cylinder, lets see.....hmmmm 3 hrs!!! still nothing.....