I have a 1989 f150 that I have owned for the whole summer and I havnt had any trouble with the brakes.. today they got really squishy and of I hold the pedal in the right spot u can hear an air sound, and when this happens the engine rpms increase... thats making me think brake booster but ive done some of the test like hold pressure and start and the peddle sinks a little bit like it should. Any ideas.?

 

I should also include that that rear antilock and parking brake light are on

 

I assume this truck has speed control? If so you are probably hearing the vacuum dump valve for the vacuum servo. The dump valve is located near the brake switch on top of the brake pedal assembly.

IIRC correctly if both brake warning lights are on you should verify the brake master cylinder is full.