Brake Problem

[mattpiotr]

I have a 1986 GMC 2500 with the JB7 Brake Code. Last fall I put new shoes on the rear along with a new brake booster. Just last week my brakes were feeling spongy and going almost to the floor before they would grab. I replaced the master cylinder thinking it was that, Now everyonce in a while if i slam on the brakes the pedal sticks to the floor. and the brakes still go to the floor before they catch. With the truck off, if I pump the brakes up I get a really stiff pedal. I replaced the vacuum line and did not seem to help. Not to sure what to look for now. Maybe the proportioning valve? Let me know Thanks.