This is an update on my original post. I've re-adjusted the brakes until I can hear all the shoes lightly scraping the inside of the drums. I've bled and bled and re-bled the lines until I'm very confident there is no air left in the system. The problem still remains. To summarize, I have to pump the brake pedal to get any brakes. Then, as long as I hold the pedal tightly down, the brakes remain firm. But, once I let off on the pedal, within a second or two I have no brakes again...the pedal goes to the floor when pushed. To get the brakes back, I have to pump the pedal again. I've noticed one thing that I think is a clue to the problem. If I don't let the brake pedal quite all the way out, so that the push rod on the master cylinder doesn't ever come completely out to its maximum point, then the brakes remain good. But if I let the push rod come completely out as far as it will go, then I lose the brakes. It is a new master cylinder, so I thought perhaps it is defective. I replaced it with my old one and the exact same thing happens. It almost seems like I need to shorten the linkage to the push rod so that it cannot come all the way out, but there is no way to do that. Any ideas?
