1998 Chev C1500 spongy brake pedal

[GMC Man]

1998 Chev C1500 5.7L Auto,With anti-lock brakes, NO CEL, SEL, or warning lights of any kind.163k mi.
  I recently replaced the rear brake line because the original one had holes rusted in it and leaking brake fliud. A spongy brake pedal was observed before this. The line was bled in the usual way, farthest from master cylinder first etc.  I also performed a rear axle brake job and adjusted the shoes to as close to the drum without contact as possible. Drove the truck applying brakes forward and in reverse to adjust the usual old way.The front brake pads have been recently replaced and rotors turned.I still experience a spongy brake pedal and in a "panic brake " situation the brakes seem to be weak. Is there a test for a weak master cylinder? Am I bleeding the brakes properly? Do Anti-lock brakes require special bleeding?

[stoneconel]

sounds like you still have air in lines, i would try bleeding them again.  we had to pump almost two quarts thru my brothers van after replacing the rear wheel cylinders to get rid of the spongy brake feel! i think air bubbles stick to the insides of the lines and inside the cylinders.

[Transman]

Be careful on this one, you may need to have a scanner on it to properly bleed the system. We had one come into the shop recently, the customer had replaced a brake line then replaced 4 master cylinders thinking the problem was there. We hooked up the scanner and bled the abs system then they worked perfectly. There is also a tool for manual bleeding that simply pushes on a pin on the abs control valve that supposedly will allow it to be bled, I have had very little luck with that technique though. Keep us posted, Transman