1996 Chevrolet Silverado

Brakes problem
1996 Chevy Silverado V8 Two Wheel Drive Automatic 130000 miles

My Chevy has a JB6 brake system, the pedal travels nearly to the floor befor grabbing disk/drums. I am trying to determine whether it is the brake power booster, the proportioning valve or the power boost system. The master cylinder is new, I have drained the entire system and used new brake fluid, and bled the brakes thoroughly. When I bled the brakes with the vehicle off, the brake pedal gets tight, however once the vehicle is turned on then the pedal becomes loose and has 2-3" travel, it just doesn't seem to grab immediately when depressed. Can the proportioning valve malfunction or the hydro power boost fail. I am leaning more to the power brake booster itself. Any suggestions?
March 8, 2010.

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