I had stop real quick and hard from someone running a red light in my 94 Sonoma Highrider 4x4. So I stomped on the pedal real hard to stop and the very next time I had to stop, the pedal went to the floor and I had to pump the brakes to slow down and stop. They never recovered.

No leaks from the wheels cylinders, master cylinder, lines, or booster. No brake, SES or Anti-Lock lights are on.

In the off mode, I can pump the brake until the pedal is hard and it holds. Once the engine starts, bam, to the floor. So I replaed the MCylinder with the same results. I bleed the system from the LR -> RR -> LF -> RF, twice. Same results.

My next move is the booster but the symtoms don't act like a typical booster. I did notice a vacuum (hiss) sound (not a loud sound, but loud enough) in the cabin when I first press the on the brake pedal.

Is the booster the logical part to replace?

All answers welcome.