One other thing to check, from my experience working at a field that rented Mirages, see if the valve seal has come unseated from the valve cap. At least on the later model ones we rented the valve seal was pressed into the valve cap and the center crimped in to hold it. Sometimes the seal would wiggle out and then all kinds of weird things would happen. Generally low velocity and leaks, but burping was also sometimes a symptom.
Nunquam emere quid potes facere