Thanks for the insights, everyone.
I usually drain the tank twice a week so didn't think it was a water problem, but after staring down the machine from across the room, realized it was sitting a lot less level than I thought (whole basement is pretty sloped, the corner it's tucked worse)...
Couldn't find an answer for this.
I have a ValAir 4gallon compressor running a Gravermach AT. Have had both a couple years but only recently thought about how quickly the compressor needs to refill. I'm getting about 2.45minutes between the compressor kicking on while doing fine shading, so...