Chevrolet: Brake System no Pedal force, chevy 4x4, rear brakes

Question
Harold:

Hoping you can help me out. I have a 1973 K20 Chevy 4X4 pickup 3/4 ton.  About 2 months ago the forward resevoir of the master cylinder got real low.  I assumed I had a pinhole in a brake line. I added some more fluid to bring the level back up and brake pedal came back to solid.  Friday on way home I had no pedal again and opening the master cylinder found the front resevoir to be almost dry.  I added fluid but no brake pedal this time.  I am assuming I have air in the lines now so I know they need to be bled. Question, I dont see any fluid leaking out anywhere and yet the front resevoir was just about completely dry. Thats alot of fluid to be going somewhere. I still have a very minor amount of brake force available at the rear brakes but the front brakes are not active. Front disc rear drum. (Truck has Power brakes)

Any thoughts? Could it be the master cylinder leaking internally into the brake booster? One of the rear brakes was acting a little funny since that first incident of low fluid. It is more grabby than the other 3 wheels. Would that mean that the shoes on that rear brake are contaminated by fluid making that brake grabby? Just me guessing.
Your help is appreciated as I cant spend 500 finding out what the problem is, its only a 2000 dollar truck and that was 5 years ago.
Thanks Allan

Answer
It could be the master cyl. leaking. If it is you should be able see fluid at the rear of the cyl. where it bolts onto the booster, but I don't think tha this is the problem.
You said that one of the rear brakes is grabbing. The wheel cylinder is probably leaking.
You probably have anothe leak somewhere in the front also. It doesn't take long to squirt out all the fluid when its under pressure.