Instead of diagnosing the problem, you introduced a whole bunch of new variables into the problem by replacing random parts, most of which won't cause a low pedal. In particular, the power booster won't cause that but a misadjusted push rod on the replacement can. It IS possible to damage the master cylinder the first time the pedal went to the floor when the drum came off. Did you bench-bleed the new one before installing it? It wasn't a used master cylinder from a salvage yard, was it?

Have you tried driving the truck since installing the new parts? GMs have a characteristic where the pedal can be pushed to the floor when you're standing still but the wheels would be locked up and skidding if you were on the highway. That is normal for them but it can be misleading. A lot of people have spent a lot of time looking for a problem that didn't exist.

If the pedal really is too low, there's either air in the line yet or a rear brake isn't fully adjusted up. That can be misleading too, especially on trucks. If the shoes are not centered in the drum, one can start to drag first when you manually adjust them up and give the illusion that both are adjusted up all the way. I like to take the drum off, turn the star wheel adjuster up, then try to push the drum back on. I have better luck doing it that way than by adjusting the star wheel with the drum installed.

Did you install the shoes with the shorter linings toward the front of the truck, and the longer linings toward the rear?

Answered by caradiodoc (expert)
17,655 answers provided