72 Demon, manual drum brakes.

My master cylinder was leaking from the top. I got a new gasket for it and bent the spring to hold it tighter, but it still leaked.

So I put on a new master cylinder, and it leaked from the top also! I bent the spring to hold it firmer, and it still leaked. Then I got a piece of rubberized gasket material and cut it, and put it between the gasket and the lid, making it even tighter, but it still leaked. So I cut another gasket piece, and now have 2 of them under the main gasket, and it still leaks, but only slightly.
What is going on here? Is it just a defective design or what?

Sure sounds like you've about everything you can think of. Could you run the fluid a tad lower? EDIT I think I'd toss all your gaskets,get a new one & try less fluid with your tightened bail wire and a different cover.

Sounds like you have a bad Master as its building pressure in one of the resevoir's which it shouldn't do.

Try removing enough fluid so the level is about 3/8" to 1/2" from the top. Also remove all of the gaskets you added. Those master cylinders will spill over if they are filled too high.

The master cylinder reservoir should not be getting pressurized. And the only way it can get pressurized is if the piston in the master cylinder is by passing. What kind of m/c did you replace it with? A rebuild unit from napa or something similar? Because those rebuilt units are notoriously bad/defective.