I assume you have an oil line going to the gauge if you could blow air into the gauge? I'm don't recall the the set up on a 77 FLH.Do you also have an oil light?
By what you're saying I would guess your gauge is bad.On newer FLH's the sending unit has 2 wires on it,1 for light,1 for gauge.There is no oil line to the gauge.Was this a gauge add on in the tappet screen hole?
tourbox