I have a 2003 GMC Envoy and when I hit the brake pedal I hear a hissing sound. (Like air leaking) My brakes work fine and I stop but if I then put more pressure on the brake pedal it will go to the floor. Any ideas?
If the pedal is going to the floor, I suspect a master cylinder. The hissing sound is probably pressure being bypased within the master cylinder.
I would get it to a good mechanic you trust as ASAP.
The hissing noise is coming from the vacuum brake booster. There is a internal failure. It will need to be replaced.
It is not the master cylinder. You will not lose your brakes but you might lose the power assist.
take it to a shop asap
you will NOT want to be trying to stop, and lose all the power brakes and have to literally stand on the brake pedal to stop. especially if someone else borrows your car. this could get you or your passenger hurt.