Rear Brake dragging?

'86 700, 20K miles

I've noticed that the rear rotor is too hot to touch after a ride, but the fronts are cool to the touch. This is even after doing a test-ride where I don't use the rear brake at all, but just the fronts.

I'm guessing that the rear is dragging for some reason.

I know I have a ton of air in the rear line, due to (I think) needing to rebuild the rear master. When I go to bleed the rear, it just burps air seemingly for ever.

I think the rear master's seals were damaged by heat from the exhaust. The brake linkage was hitting the right exhaust pipe when released. I've since adjusted the linkage so that it no longer contacts the pipe when released, but it was against that pipe for a good 1-2K miles of my riding it, and who knows how many of the previous 17K on the bike.

The rear master doesn't leak at all, though, just tons of air in it. Previously, I tried to order a new piston-set for it, but that was on backorder for a month or two, and then BikeBandit cancelled the order (not available). I'm pretty sure I could get just a seal rebuild kit from a dealer, though.

I'm guessing it's probably heating up, and the air is expanding due to the heat, causing the rears to drag.

Anyone have any other suggestions? My next steps are just going to be to pull the master and tear it down, and see what condition it's in. Same with the rear caliper (making sure it's not frozen).