This is the first time I have ever heard of an air brake system's parking brake valve called a a parking brake plunger. It gives me visions of a brown rubber toilet plunger mounted on a wooden stick
But I digress: This parking brake valve, ID'd for the usually by the large square yellow plastic knob mounted to the shaft, can leak internal air from around the shaft and is usually audible to the operator. Replacement used to be the only recommended fix.
The relay valve is on the service brake side of the system and should not affect the parking brake system, AFAIK.
Semi-trucks often have a quick-release valve mounted in-line between the parking brake release valve and the spring brake cans. If you are near a semi and hear the loud pop/swoosh when the driver is parking, that is the air exhausting from the quick-release valve as the spring brakes are allowed to expand and set the brakes. Sounds similar to our air drier's popping off.
I guess the manufacturers did not want Class A's with diesels and air brakes to be any noisier than they are already when parking in a campground?
In the distant past I have had to change out the parking brake valve twice to get one that did not leak.