Don't forget that there is a suction/vacuum relief valve in the system. This is what controls the head of mercury (inches not hectors pascals) in the system and is adjustable. There is also a little gauze filter in the instruments themselves which if the aircraft is fairly old can become blocked and cause the instrument to malfunction. Remember air is drawn through the AH and DI by the vacuum pump through a filter usually mounted on the inside of the cabin and exhausted by the pump in the engine bay.
Some vacuum pumps have only a life of 500 hours so the MO should be looking at this and be changing the filters regularly.