First, unbolt the master cylinder to check if brake fluid is running into the power booster. That's the only place it can go and not be seen.
Next, if you pushed the brake pedal to the floor at any time, there's a good chance the master cylinder has been damaged. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Running the pedal to the floor runs the rubber lip seals over that crud and rips them. That results in a slowly-sinking brake pedal or a low pedal, and that often doesn't show up until two or three days later. To avoid this damage on any vehicle, never push the brake pedal over halfway to the floor unless the master cylinder is less than about a year old.
The third problem only applies to GM front-wheel-drive cars. When the brake pedal is pushed over halfway to the floor, a valve trips in the master cylinder to block a front port and the opposite rear port. No brake fluid will flow to those two wheels, and you won't be able to bleed any air out. If you need to replace the master cylinder, don't push the pistons over halfway when bench-bleeding the new one so the valve doesn't trip. Gravity-bleed at the wheels. If the old master cylinder is okay, the only way I've ever found to reset that valve is to go to one of the wheels that isn't flowing any fluid, open the bleeder screw, and give it a short, quick burst of compressed air, then let it gravity-bleed.
Tuesday, October 20th, 2015 AT 5:19 PM