Brakes Feel Like Slipping at the End of Stop

I have a 1997 Ford Taurus. No ABS. The car is not driven often or very far, maybe 20 miles per week. During braking like for a stop sign at the end of the stop the brakes kind feel like they are slipping, and there's a burping noise. The car does stop without a problem (at least for now). After the stop when I take off again it sounds like there's air building up again just to be released or burped out at the next stop. It doesn't happen every stop but probably 6 times out of 10. I read about vaccum from the intake manifold to the Power Booster but am not sure if that 's what the problem is. When in park with the engine running brake pressure is constant with no loss or pedal depression. The pads are fine, no loss of brake fluid. The fluid has never been changed. Any Ideas are appreciated.

Thanks

Danny-