Prometal is dead on correct with the bleed procedure. One other thing I heard works is to jack the vehicle from the frame and let the wheels hang as low as possible to get them below the master cylinder. From what you describe it does sound like a faulty master cylinder though.....It does happen...
Dot 5 is hard to get the air out of. Is that what you are using?
Are you sure the stroke of your brake rod in the master cylinder is correct? If it is preloading the master cylinder, you will not get a good bleed.
Under floor mounted master cylinders are particularly difficult. The problem is getting enough fluid flowing fast enough to push the air bubbles out before they back flow into your lines toward the master cylinder. Either a vacuum bleed with a large reservoir and a good means of keeping the master cylinder full of fluid is necessary, or you need a proper pressure bleeder. A good one can be quite expensive. This may be where you want to go to a brake shop that has the expensive unit and just pay labor, or search out a friend with a good pressure bleeder.
I have gone through this several times. In one case I had a leak that was almost undetectable. Look at your fittings very closely. Even the slightest leak will put a bubble in your line.
I had some similar problems with my 56 Ford that I put a power brake setup on. The rod between the master & the booster has to be adjusted to just the right length. In your case, I would adjust it out 1/16" - 1/8" then try it. If that rod is too long, it will build up & drag the brakes. Too short & you'll have to pump 'em. Same thing goes for the rod to the booster from the pedal.
Lance
53, 56, 64 F-100s
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