Got out there today and checked the vacuum for the booster, the lines, etc. I noticed the bottom of the left rear drum was a little wet. Removed the drum and it was a mess. My guess is the wheel cylinder has been leaking for a bit, but it's only gotten worse. Now, whether that is the sole cause of the pressure loss, I'm not sure but the seal was pushed off on one side and the shaft was fairly wet. I wiped it all up, used a screwdriver to lift the seal while he applied the brakes with the engine off. I could see it squirting just a little and the piston wasn't releasing at all. After a couple more times it started flowing quite heavy. Obviously the cylinder isn't functioning like it should, but that wouldn't have anything to do with applying the brake with the drum removed, would it?
I would think even with the drum off and applying the brake, that cylinder should not leaked regardless. Not to mention there already being signs of it leaking previously. That was the only wet wheel.
Do you see a leaking wheel cylinder accounting for the pressure loss described in the original post? I've already warned them after the cylinder is replaced, an issue could still be present.