1991 GMC C1500

Brakes problem
1991 GMC C1500 V8 Four Wheel Drive Automatic

slow brake pedeal to botton, bled system and brake pedal goes slowly to botton. Replaced master cylinder twice and bench bled outside, then bled all four. Puched on pedal and slowly went down to floor. Have no leaks. After bleeding and gravity bleeding know rear brakes won't bleed and pedal goes to floor slowly. I know I got all air out. Help! Could it be a proportioning valve? Help! Many thanks to the Master ASE technicians that answered my question, I forgot to say that I Bench bled the masters, I know all the air was out. HELP!
July 16, 2009.

Did the new master cylinder come with a couple hoses and plactic screw things? If yes use these to bench bleed master.

Jul 16, 2009.