DD15 constantly building air and purging every 20-30 seconds.

[SquareOne]

THAT leak can 100% be your issue. The same very minuscule on my Cascadia was causing the same issue. Replaced the fitting and all was good.

Never actually showed on the air gauge because it was not leaking at the tanks. I'm assuming the governor is sensing the small drop in pressure at the line and causing the compressor to start building for a short time (although the tanks are full). Mine was popping the air every few minutes before the repair.