ISC cm 2250 fuel system issues

[dodges1515]

2012 FRT. M2 isc cm2250 10,000 hrs offroad. I have been fighting intermitten fuel drainback issue for over a year.No patterns, just on any given morning it may or may not start. Acting up more frequently now.If you remove fuel supply tube to piston pump it takes a while of cycling key and cranking engine to run gear pump to get fuel to piston pump and when it does come it looks like pink champagne (offroad fuel) lots of small bubbles.After cruising around fuel troubleshooting in cummins quickserve and doing my best at running their tests without all of their correct service tools at one point I had two see through line hooked to the two male quick connects at the ecm cooling plate with each line submerged in 3 gallon containers half full of fuel,the suction side line container placed slightly lower than the pressure side container.After running the electric lift pump while viewing fuel flow i noticed that on certain occasions after shutting lift pump off,fuel would siphon back through to suction side container until it emptied the pressure side container.
Cummins calls the check valve that is in the lower ecm cooling plate port a "checkvalve" and in some instances they call it a anti-fuel drain back valve.Now after removing that check valve it appears all it does is check fuel in the cooler until gear pump suction unseats it and pulls fuel through cooler rather than drags fuel through the electric lift pump,(I think)it cant check all the fuel because of the banjo fitting that goes to electric pump would allow fuel to pass in front of the check,(i think).The check valve that is in the lower port definitively checks. I removed my lift pump and it does not appear to check if you blow through either way.
I purchased a new lift pump and it does appear to have a check of some sort, whether it is naturally the result of the mechanical properties of the pump or a actual check valve I do not know.
Also if the suction side of the system becomes "system drainback" in my case ,where does the air come from that displaces the fuel as both system suction and return and submersed in fuel in the fuel tank?
Any feedback would be appreciated.
Lift pump passes flow test.Gear pump passes engine cranking psi test and engine running psi also.No fuel leaks to atmosphere.