soft brake pedal on 1991 b2600i pickup
 

My brakes were working perfectly. No leaks, nice high pedal. Front pads were getting thin, installed new ones. Never disturbed the hydraulic system. After that the pedal was very soft, as if air somehow got into the system. Bled brakes, got no air, no improvement, bled many times with both a vacuum system and the old fashioned two person pumping method. Still pedal would go gradually to the floor with the engine running. Felt like a bypassing master cylinder. Replaced M/C with bench bled new one. Same result, pedal will go slowly to the floor with engine on. Bled about 1/2 quart thru the system with no improvement. Adjusted rear shoes closer (slight drag) Pedal improved for short time but went back to soft after a short time. This truck has ABS; could that have some effect on this problem? Both brake warning lights go out shortly after turning on the ignition, which I assume is correct. Can anybody give me a clue how to fix this situation? I'm thinking power booster pushrod maybe needing adjustment, but this problem started with the original master cylinder which had been working perfectly before I changed the front pads.

My 92 b2600i does essentially the same thing. Did you ever find out what is causing it on your truck?

Did anyone ever solve this problem? Same issue here.