We have a 1991 g3500 bus here at school that we use as a shuttle. It has hydroboost brakes on it. Having an issue with them as of lately. When you come to a stop sign and you press the brake pedal, they are super weak and the pedal goes almost to the floor. But if I quick pump the pedal once, then press the pedal, its solid and it stops great.

I know there is no booster for it, so what could be the problem? Like I said after a quick pump, its solid and stops great...

no leaks, fluid is full and in decent shape..pads do need to be replaced soon, but still have some left..

Thanks a bunch

 

Master cylinder could be getting bad or back brakes need adjusting.