Trying to diagnose a problem on the 2004 Grand Prix GTP Comp G.

Every so often, the car seems to lose a bit of brake boost... its not a lot, but its enough to be real irritating and if your not careful, it easy to misjudge your braking distance.

It seems that once you get off the gas, the engine will upshift, and so the car is losing idle speed quickly, if it is not already nearly at idle. Meantime, you are applying a constant pressure on the brakes, just scrubbing off speed, and when the car is nearly at idle, then the car suddenly stops slowing. You have to immediately push more to get it to stop.

I immediately thought I had a vacuum leak, but I would expect to get an engine code. I can't imagine its a bad booster... as I would think that would essentially be a vacuum leak, as well.

I'm hoping its not master cylinder, which at 114K would seem a bit early (my '99 Bonne has 230K on its master cylinder). The front brake pads were replaced recently, and to my knowledge the fluid was never run low.

Any thoughts what could be causing this?

Vacuum leak is the first thing that comes to mind. Another thing to try, with the engine running and sitting in park, pump the brake pedal a few times then push and hold it down. While you are holding it, does it maintain position, or slowly drop to the floor?

When casually using the car, holding the brake, it does not drop to the floor. I'll need to test this car again next time I'm driving it.

I've always been iffy with this technique, as I'm a big guy and can slowly mash most pedals to the floor.

No mashing required. Just apply the same pressure you normally would to stop the car.