Have a 93 isuzu npr just replaced wheel cylinders on front right and right rear and while there the shoes. This truck has drums on front. After repair bled the system and can't get a solid pedal with the engine running. There is not any leaks. After the bleed the pedal will get solid like it should have sufficient brake application. Although once the engine is started pedal falls and gets spongy just like air in the system. Brake effort on the pedal to stop or hold in reverse takes a strong foot, going forward it will stop though it seems that pedal is too low which does not feel right. What is the proper bleed procedure on these trucks? And could the master cylinder be causing these problems?