I have 1998 chev. 1500 p.u., Brake pedal was spongy, replaced 2 rear wheel cylinders and pads. Bled off lines,still very spongy. replaced front pads, re-bled lines, still very spongy. I have re-adjusted rear brakes and re-bled lines several times. The brakes actualy work good but pedal remains very spongy and close to the floor board. Pedal pumps up hard with engine off, gets very soft and close to floor board when engine is started. Can not detect any leaks, master cylender does not leak, vacume booster seems ok, do not hear any leaks. Can you give me any idea where to head next. Thanks for your help,   Bill B

Optional Information:
1998 Chevrolet 1500 4.3

Already Tried:
replace wheel cylinders,brake shoes,front brake pads re-bled line several times.re-adjusted rear brakes