I have a 69 f100, with drums all around. When i got it, it had sat for a couple years, and i replaced both rear wheel cylinders, and put all new pads and hardware in the rear, but didn't on the front. After that the brakes worked fine for a while. Eventually i had to start pumping the brake once to build up pressure. Now i have to pump a couple times to build up pressure, then i will be able to brake fine, but as soon as i let up on the peddle for a couple of seconds i lose all brake pressure and have to start over. Any thoughts on what this could be? I can't seem to find any leaks in the line, or in the front brakes. Anyways, thanks in advance!

 

Sure. Master cylinder has an internal failure.

 

Alrighty, thanks. Thought it might've been that but didnt really think It through.