Just a thought Sam, I had some irratic issues with mine also, even after the rebuild. I was informed by an older gent that my airhorn mine be slightly warped. He said to remove the airhorn and lay a straight edge across the bottom of the horn. If there was any gap, usually in the center area, I would experience problems due to leaking. Warpage of the airhorn is due to over tightening of the bolts over time.
I performed this test and he was correct. I carefully filed the perimeter edges of the bottom of the airhorn until the gap was gone, you need to use a large file. Once the gap is gone, thoroughly clean the airhorn of metal filings. Also when reinstalling the gasket between the airhorn & the base, lightly cover the gasket with a white lithium grease on both sides. This will also help to eliminate any leaks and do NOT over tighten the bolts. Good Luck!