4x4 brake problem

[jdb368]

I have a 1973 Chevrolet 1/2 ton 4x4 with 44 inch tall tires. It has a new master cylender, and power brakes. The truck has front disc and rear drums. I have put a new brake valve and a new front disk calaper. The truck won't stop like it should. If you push the brakes to the floor the back will lock up but the front don't seem to help the truck stop any. I have tryed the truck on stock 35 inch tires and the truck still doesn't stop good. Does any one know what the problem void be. I have already changed out A few lines, flushed the system with new fluid, and have bled the system several times