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Each of the rear air brake chambers on your coach have 2 rubber diaphragms and a spring inside. One diaphragm activates the service brakes when you press on the treadle. The spring applies the same force to the same slack adjuster as the service brake when parking is applied (knob pulled). The other diaphragm, likely your problem, is pressurized when knob is pressed in. That diaphragm retracts and holds the spring back from the service brake. Likely, you have a pinhole in that diaphragm and air pressure leaks off when park is released.
If you securely chock all wheels to be sure coach can't roll, press in park brake knob and you should be able to hear air escaping from one of the rear brake chambers. It is good practice to replace both chambers when one fails. It is very dangerous to attempt to replace a park diaphragm in a used chamber. Fortunately, replacement chambers are not expensive. Would be a good time to replace the service diaphragms as they are cheap and will be off while the chamber is off.