1990 Ford Bronco

I have a bronco 2, and when I use my clutch pedal too much it will begin to lose pressure. Then it won't allow me to change gears because their is no pressure to depress the clutch pedal. For instance, if I am parked and I pump my clutch pedal, it will pump normally for about 5 pumps and then lose all pressure and I can't shift. The fluid level is fine. If I am driving and this occurs, I will shift from 2nd to 3rd and all I have to do is tap the gas and it will shift gears almost like there isn't any pressure again. I think it might be the clutch master cylinder that needs to be replaced.