Thanks to information from this group, I had installed a gravity powered
recirculating loop in my hot water line, with a brass check valve
(flapper type) to prevent backflow. It worked very well for years, but
now the check valve appears to be stuck in the closed position,
preventing circulation, and we have to wait for hot water again.

Is there a good way to fix this valve, short of replacing it? It has a
cap on top, but apparently they put that on at the factory so tightly
that it cannot be removed. I tried some gentle tapping, then some more
vigorous tapping, but that didn't help. I thought of applying heat, but
I don't want to undo the soldering.