Try bleeding each wheel cylinder especially opposite the problem corner by removing one drum, pumping the brake pedal until the wheel cylinder pistons are out as far as possible (without falling out of course) then have someone hold the brake pedal down on the last pump, open the wheel cylinder bleeder, push the wheel cylinder pistons back in to purge any air out of the entire wheel cylinder. You might only need to do that with the wheel cylinder opposite the locking brake drum,........hopefully.

Starting to seem to me that the locking drum is actually free of air in the line and taking all of the master cylinder pressure and working correctly. While the other side still has some air in the system and just spongy??? Just throwing stuff out there to see what sticks.......besides the one brake drum Sorry not funny, apologize good luck

Mike