It sounds like something is sticking...
I am guessing from your post that the wheel cylinder has been replaced?
If not.....I would replace both rear wheel cylinders....they are not expensive.
IF you replace them, I found that the nut on the metal brake line on the back side of the wheel cylinder can stick, causing the brake line to turn when you turn the nut.
Easy solution.....Remove the 2 small bolts that hold the wheel cylinder to the backing plate and then you can spin the wheel cylinder on the brake line (with a wrench on the brake line nut).
As 12ounce mentioned, the automatic adjusters can stop adjusting after a period of time, but that seems to cause the brake to not work.....but, again, it seems that the one side is sticking for some reason.
I mentioned the wheel cylinder because it is something that is cheap....and can do this intermittently....and you cannot see inside it, like you can all the slide & pivot points in the mechanism for the brake assembly.