I don't remember a "pup" on the parking brake assembly, but I do remember the quick release valve, which I believe releases the air pressure on the parking brake assembly, which we replaced, that I believe had an o-ring assembly. Perhaps that's what the mechanic was talking about. That didn't solve our problem, but it's what the mechanics tried first, because they initially thought that was the problem.
In any case, I'm glad that solved your problem. Ours was a leak between the outer case and the inner assembly on the rotochamber, letting air escape.
BTW, here's a link to a cross-section view of a rotochamber from Bendix website. If you look at it, you can see the inner and outer diaphragms, and the spring. Air is inserted between the inner and outer diaphragms and compresses the spring, releasing pressure on the parking brake, so the driveshaft can turn. When the air is released, the spring pulls the yoke assembly and applies pressure to the parking brake drum, which stops the driveshaft from turning via braking action. That's why the parking brake is spring-loaded on and air pressure applied to take the brake off.