Brake Help Silverado 1500. 2008
 

So my Brake pedal went to the floor. Checked and no leaks, with the engine switched off I can press on the pedal and it is solid. Replaced the Booster (vacuum only). Still the same. Master was changed 2 years ago.

Any ideas???

Air in your brake system.

If it has rear drum brakes, fluid may be leaking out of one of the wheel cylinders inside the drum. It’s absolutely possible for one to be leaking and still have a solid pedal with the engine off, so check for fluid dripping out of the bottom of the drum and onto the inside of the wheel. It will be most noticeable after sitting overnight.

Otherwise it’s possible one of the cup seals inside the master cylinder flipped, which could also cause the problem. The MC can be checked by removing the cap and watching for air bubbles as a helper taps on the brake pedal. You should see 1 or 2 air bubbles from each bore - any more than that, or bubbles from one bore and not the other, indicates a problem internal to the MC.

Just FYI, if the booster fails, the pedal will be very stiff even with the engine running. The only reason it’s there is to make the pedal easier to push.