Only specific pneumatic fitting suppliers will have a shouldered plug which would need a dowty washer, (or Best fittings). Plumbers merchants should have a tapered plug.
Personally I wouldn't recommend using a tapered plug as it could stretch the threads if you kept on tightening it a bit too much.
Sounds like the gauge has bust inside to me as they can fracture inside on the bourden tube.
Hope you sort it but I cant see it being much of an issue to sort.
Last edited by DEAN C.; 26th June 2015 at 09:21 AM.