My brakes are going out, but i dont see any leaks on or near the master cylinder.?

my brakes slowly started to get squishy, and then all of the sudden they went all the way to the floor. I topped the fluid off, and was able to pump the brakes for full power stopping. Now the only way i can stop safely is to pump them a few times. there are no leaks near the master cylinder, and the fluid level has stayed the same since i stopped it off.....1995,chev,k1500