I have a 2002 GMC Sierra 1500 4wd extended cab pickup. I was driving in 4wd auto yesterday and my brake pedal went to the floor, it stops the truck but its low to the floor. Anyway i decided to put the truck back into 2hi and the brake pedal was fine. It did not go to the floor and felt normal. What is causing this and how do i repair it?
if the 4x4 is causing a vac. leak may effect the brake booster and the running condition
if the brake pedal goes to the floor only once and then pumps up right away i would check my front wheel bearings for play, they can push your front pads apart causing excessive travel before pumping up again, noticed this myself in my 2000 especially cornering or pulling away from the curb, something to check. also a vacuum loss would give a hard pedal (no assist)
There is no connection between the 4WD system and the brake system in that vehicle; I think you just experienced a coincidence. (In some newer vehicles, there might be a connection through the traction control system. But not in 2002.) A brake pedal that sinks to the floor, but returns to firmness when pumped even once, is a classic symptom of a failing master cylinder. Go out and sit in the truck, and press on the brake pedal. Doesn't matter engine on or off, or whatever. Pay close attention to your foot: Is the brake pedal slowly sinking to the floor? Give it a full minute with your foot on the brake. It's possible that you've got a leak elsewhere in the system. But I'd bet on the master cylinder. Don't wait, though. Get to a mechanic before this thing kills you.
