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.
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