Thanks Jim............

Ron...............good suggestion, I'll remember that one for my F5 where I'll have access to the full air system in the nose. Unfortunately, the current model has the whole system apart from filler valve & pressure guage housed in the wing centre section, so it's totally unaccessible...............re-plumbing to fit a mechanical shut off valve thats accessible via the front hatch would be a right pain in the A**E !!