Overheating rear end

Imagine there is air in the rear system, close to a heat source; could that air expand enough to energize the wc's?
We know that drum brakes are self energizing. So, If the front shoe moves out enough to catch on the spinning drum, it could drive the rear one into the drum, just enough to drag.
By the time you get the corner jacked up, to check it, the air might have cooled, the w/c retracted, and you'd never catch it.
I mean just throwing stuff out there.

Easy test is to back off the rear adjusters so there is no possibility of dragging.
If you suspect air, I imagine a good place to look for it, is in the crossover line where the fluid goes uphill over the differential.

When I was younger, I would clamp the rear flex hose. But, if the line is old, it can come apart inside and act like a slow-release check-valve. That actually happened to me once on a front caliper. Sat on the side of the road for a bit, the brake released, and I drove straight home. The point is, that I don't clamp the flex hoses any more.

To accurately check for air, in hard cases, what I have done in the past, is to remove all the drums, and all the shoes, then C-clamp all the w/c's in such a manner that the pistons cannot come out. Thus the pedal, when you step on it, is high and hard, like a rock . Two or three rapid stomps on the pedal, should bring the pedal even higher. When you take your foot off the pedal, and wait for a few seconds, the fluid will return to the reservoir, and the pedal drop to normal height. There should not be any sponginess in the pedal.

I'm not saying that you need to go thru this procedure ..... yet. Because it is so time consuming, it's kindof a last resort. But, it is definitive. I've never had to do a drum/drum car; it's always been disc/drum, or disc/disc dual M/C system, in my line of work.
In your case, since the front is normal, and you have a single power-piston in the M/C, I imagine doing just the rear, might be sufficient.

One more thing worth mentioning;
is that hard lines sometimes rust from the inside, from moisture accumulation, and the rust forms in the lowest part of the system. I have seen that rust pocket also act like time-release check-valve. In my case, the M/C had enough hydraulic pressure, to push the fluid out, but the return springs were slow to return it thru the restrictive rust pocket. I my case, I just cut the bad section out, and stabbed in a jumper, all properly double-flared of course.
You can see this in action by removing the rear drums, ONE atta time, and watching the action while a helper slowly presses and releases the pedal. The normal action should be smooth and retraction as fast as application.

and finally, what if the front brakes are not braking, and the rears are doing all the work?