When the topic of air intrusion comes up, it seems like there are generally two types of responses. The first is people will suggest common sources of air intrusion and parts to replace. It's pretty much a guessing game not really based on evidence. The second is the replacing return lines with clear hose and looking for air bubbles. I don't really like the sounds of this for a couple reasons. You have to disturb a bunch of stuff to install the clear lines, and it seems time consuming. So...can the return lines be pressurized and each connection sprayed with soapy water to pinpoint the source of the problem? You could regulate an air source down to 5ish psi and connect it to schrader valve. Would you have to clamp the return line leading to the tank to hold pressure? Would the lift pump hold pressure on the inlet side (not really sure how they work and if they'd act as a check valve for this). Any risk to damaging the diaphragm in the lift pump? It seems like the regulator, and adapters and stuff to do this could be purchased fairly cheaply. Any thoughts?