a bucket of water with air attached and to look for bubbles. This would find a leaking O-Ring. However, the seal to the lower plenum is provide by the "insulators". To check this without the Plenum attached requires you to make a fixture that screws down to the lower plenum & you apply air and listen for leaks. Got the idea from a guy on the forum. Maybe he'll post the same pics that I used to make mine.