Problem solved.
Wheel has slight drag, but a 20 minute ride yesterday (it was 105 outside so not far) yielded a warm but not sizzling hot front rotor.
I still cannot say for certain what the problem was. The piston in the master cylinder was FULL of crud (see pic). Don't know if this was affecting piston travel or the tiny bleed hole in the reservoir was plugged up. I suspect one of the previous owners mixed Dot 5 with the factory Dot 4.

This after rebuilding the front caliper and replacing the brake hose. And yes, the rotor is not warped. Thanks to all for input.

 

Update:
I must have received a counterfeit part because it was missing all the bolts and pieces, dust boot, etc. It was ugly. Just didn't look right as it had rough edges, etc. The piston was extended and refused to be pushed back in, even in a vise. I sent it back and received the right part with all the pieces this time. It went on with no issues yesterday, but of course I have no fluid coming out of the master cylinder now. I'm guessing I have a clog.

I removed the brake hose and pulled the brake lever. Hardly any fluid came out. I'm a novice mechanic, but that doesn't seem right. It should push a decent amount of fluid out, right? Plus, almost no air bubbles as I pulled the lever on an empty, connected system. I tried pumping slow and fast.

Also, do you all use Loctite blue instead of the Harley brand thread sealer? I've seen differing opinions around the internet.

 

Did not know you could use DOT 4 on older bikes. Thought it had to be DOT or DOT 5. That said, if the fluid was mixed, you need to disassemble the brake caliper, the brake lines and the master cylinder and flush the components so that there is no residue and completely clean. Then refill with fluid.

I haven't used blue locktite on brake thread components, just the automotive type that is available at any auto parts store.

HD was using the Wagner Lockheed style rear brake master cylinder thu 1979.




Check these areas for blockage. One of them is a very small hole. A diaper pin barely goes thru it.



And I notice it was suggested that original master cylinders cannot be serviced. They can and the one in the pic was. It was resleeved because the inside was too pitted. A company callled Apple Hydraulics back east does the work.. Some folks prefer the shiny Tawain stuff from places like J&P. I prefer original when it is at all possible. Yeah, probably cheaper, but the lack of quality control leaves a lot to be desired.