You are correct about the out of round brake drum, I have had both drums and rotors go out of round. It's a real pain. If it's the rotor, it can cause a shutter when you press the brakes, but if it's the drums, it can cause one wheel to lock up,especially on gravel. In my case, I thought only one wheel was the problem. when I cleaned it up and went for a test drive, the other back wheel acted the same way so I cleaned that one up too.
To avoid this problem, I usually take my back drum brakes apart twice a year and clean out the dust I take it outside and blow it all out or simply use a paint brush. I think that the accumulated brake dust gets wet when driving in rain and forms a kind of paste. If you keep the brakes clean, no paste. Unfortunately, I have neglected the cavalier and haven't cleaned out the brakes in years.