It is possible to distort the drum if it gets hot enough and then the
brakes are applied forcefully before the drum cools down. Are you
getting any pulsing while braking? I'm not sure exactly what happens
to brake fluid when it boils, but you may want to bleed the rear brake
slaves or the whole system if it has been 2 years or more since the
last bleed.

Keith O.
90 Westy Syncro "VikingWagen"