Okay, so I had the brakes done last year. There was some 'wetness' on the booster and paint coming off so I had the Master Cylinder replaced. Got the car back and put on a few thousand miles maybe...

Had a 3 car accident in front of me and slammed the brakes pretty hard and avoiding being car #4. Then a few days or a week later I left work and had super spongy brakes, the brake light came on, on the dash panel. I got home (2 miles later and very slowly cautiously) checked the fluid and it was pretty much empty, took it in and they said the driver's side front was full of air. They bled all four wheels and topped it off. I have driven it about 30 miles since. The brakes feel better than they were but maybe still soft. I thought my brakes on the 97 z28 always felt more responsive than the 99 tahoe. But I drove both today and the tahoe feels more responsive.

I went to taco bell and when I applied the brakes I heard a whistling or vacuum sound that seemed to come from the booster area. I also heard this when I let off the brakes. It seemed to only happen right when you applied or released the brake, meaning the brake pedal was fully extended not fully depressed? I cannot say however that it was even my car as there were other cars behind me. It seems as though the fluid is still appearing on the booster but no real sign of a fresh leak.

What should I do?

 

It sounds like you have a bad master cylinder (again). That brake fluid had to go somewhere and if you don't see a leak anywhere else then it probably made it's way into the brake booster. Sounds like the brake booster is bad now too... I know it's strange to suspect a part that was recently replaced, but that doesn't mean it's not faulty again.