Truck Repair: 1991 Chev. K1500 4x4 brakes, rear drum brakes, chevy k1500

Question
I'm having a problem with the brakes on a 1991 Chevy K1500 4x4. Truck has front disc and rear drum brakes.  Have replaced: all brake shoes, rear cylinders, front left rotar, and installed new (not rebuilt) master cylinder.  Have checked entire line, but can't find any leaks in line or at wheel cylinders.  Have tight pedal when truck is off.  When driving I sometimes have full pedal, but usually I have no pedal and have to pump pedal for braking power.   Have bled all air from lines.  Was told it may be the power booster but, considering the pedal is tight when truck is off and becomes soft once truck is started, I'm not sure that's right but I'm also not sure what else to check.  

Thanks for any help you can provide,
Mandy

Answer
Hello Mandy,
Usually the need to pump is because drum brakes are out of adjustment, but they usually are not intermittent.
There is the possibility of something in the antilock brake system being stuck and allowing fluid to bypass.
There is also the possibility of a problem in the NEW master cylinder. I had one with a piece of metal from the factory that would get in the check valve on the spool and make it go to the floor. Terrible feeling.

Van