I have a 1952 1 1/2 ton Chev farm truck that I'm trying to get working again. It runs halfway decent, but I have a brake problem. I can fill up the fluid reservoir and pump the brakes up, but when I try to bleed the brakes, I can't get anything to come out. If I keep pumping the brakes, I have good brakes, but as soon as I let off the peddle for a few seconds, it goes back down and I have nothing. What am I missing?