2002 Pontiac Montana. stepped on brakes, peddle went to the floor. pumped the peddle and the brakes would grab then peddle will go to the floor. brake pads and shoes good. no brake line or wheel cylinder leaks and fluid is full. one thing I did notice, is when you step on the brake pedal the fluid in the master cylinder or air is being pushed out.

SOURCE: bleed brakes but no pedal

Good! You bled the master cyl. before installation! The proper order of bleeding the lines, is to start with the wheel the greatest distance from the master cylinder. Then work your way to the next greatest distance, and last, the closest wheel. Make sure all air is removed from the lines before you proceed to the next wheel, and frequently check the master cyl. level, because, as I'm sure you know, if you **** in air at the master, you have to start all over again. I'm assuming you don't have a power bleeder, so with a helper do this with car running (make sure it's on safety stands and not going to fall on you!) Pump pedal 3 times, no need to mash the pedal into the floor, just 3 pumps 1/2 way down, hold 3rd pump at half way down level while bleeder is opened, when flow is about to stop at bleeder, close bleeder and repeat til no evidence of any air. Let me know how it goes and if you need any help.

SOURCE: 1999 Pontiac Bonneville SE 3800 ABS Brake pedal will eventually reach floor after stopping while hold car stopped. Brake pads replaced less than 5000 miles ago. Fluid level at max and does not drop. O

I would replace the MASTER CYLINDER. BLEED AIR from master cylinder BEFORE INSTALLATION. Hope this helps

SOURCE: I am drying to diagnose brake problem with 1993

yes it sounds like the problem it could maybe be the brake booster if the brakes are better at a higher idle but there is a check valve on the brake booster that cost 2.00 too replace that runs off vacume i would replace first

SOURCE: 2002 bonneville brakes sometimes push

Sure sounds like symptoms of bad internal seals in your master cylinder.