Are the shoes correctly installed? Is there a leading (shorter) and trailing (longer) shoe on each side of the car with the longer shoes towards the rear? Is The emergency brake cross-bar properly positioned between the shoes? Have you replaced the return springs and are those springs properly positioned and installed?
With the drum off, have someone GENTLY and slowly depress the brake pedal while you watch the shoes being pushed out against the springs. When the helper releases the brake pedal, do the shoes retract quickly and completely?? Look closely at the “good” side and see if you notice any difference from the problem side…
The next time it happens, pull off the road, jack up the tire and remove it and momentarily open the bleeder to see if hydraulic pressure is the culprit or is it a mechanical problem with the brakes…If you find no pressure (there should be none) then the brake has a mechanical problem…If brake fluid squirts out and the brake instantly releases, you have a hydraulic problem…The collapsed flex-line codger mentioned or a defective master cylinder that is not opening its relief ports… With ABS, each wheel has it’s own brake line coming from the ABS unit. You can disable the ABS by pulling its fuse for a while and see if that cures it…It will take a while for the ABS accumulator to bleed down so give that a few days to work…