1991 Toyota Pickup

Ok. But the pedal does not slowly leak down. It either has pressure and works fine or it goes directly to the floor, causing me to have to gear down to prevent wreck. The pedal is not stiff at all. When it is working properly theres no bleed down of the fluid. It works properly 6 outta 10 times. I never know if its goin to the floor or gonna actually stop the truck. The rear brake shoes have been replaced and no leaks(visually) that I see.

So am I correct. You say that my problem is the master cylinder. Even tho it does not bleed down when at a red light with brake depressed, per say. If this is correct I wanna thank you very much for your time.