[cknuth]

Thanks Bill and Richard for the info. I left the compressor off overnight as the project was interrupted. This morning, the slide seals were down as well - I'll take that as evidence of slight leakage in that circuit, unsurprisingly. The original dump valve circuit leak was fixed by replacing the offending push-in T with a compression fitting, beginning the gradual process of replacing all. Richard, I surmise that you use 545 instead of teflon as a thread sealant? Or is it involved somehow with a compression ferrule?
Compressor was coming on every 45 minutes with no appliance use - just now had two hours between compressor cycles including a toilet flush and a round trip of the air door. Victory lap.