Get the clear line on the return side and dont ever go back to rubber. You’ll have to replace it every few years but it is a like $2 and very easy. That is where you need to see if there is air bubbles.
A trapped small bubble is ok, but flowing bubbles means you are sucking air. If you chase the whole truck and cant find it, GM/ Stanandyne says also put clear line on the inlet to the ip to see if the ip is the cause.
Never heard of checking for air out the water drain valve. But opening it while running is a smart idea, and you should measure how many seconds to fill a quart there, then let that fuel settle out and check for contamination. Word of caution, if the ip is completely dead (noise from lift pump doesn’t mean it is pumping) then you will suck air in the drain valve and have to bleed the injectors.
8-14 psi is desirable at the ds4 ip inlet, but Like AK said, hitting zero means the lift pump can’t keep up and you are wearing out the ip in a hurry. Rig in a permanent “T” and put a gauge on the dash is smartest move. You need to see the gauge when the engine is working it’s hardest.