No Brakes

[TjBlueRusty]

This is a 1975 C-10 1/2 ton that sat for about 10 years. The easy part was getting it running the hard part seems to be getting it stopped. Heres the story. The master cylinder was empty of brake fluid. I added some on its initial drive but no brakes. I drove it slowly on some rural roads for a few miles. The right front caliper froze locking the wheel. I next relpaced both front calipers and then tried to bleed them. Started with the passerger side and after about 5 attempts I seen fluid leaking from a line going to the rear. It was rusted out. I replaced it and started the bleeding processes again. I started with the right rear but I twisted off the bleeder value. Went over to the drivers side but was unable to get any fluid. I moved to the front and got no fluid. I then tested the master cylinder and fluid shot out when I cracked open a line.
I then tried pumping fluid up from the bleeder valves. I started at the rear but it did not take much fluid and the MC reservior level never changed. Tried the right front and it took a descent amount of fluid but the MC level never changed. The left front took little fluid and the MC level never changed. Not sure what to do next. Could the Combination Valve be my problem? What's the easiest way to remove it? Looks like the radiator has to come out first. Any other suggestions?

This truck will only be driven in pastures/fields and some rural roads. If I can get just a couple of brakes working it will do.
Thanks.