Hey @ChiefK19
I have experienced this exact same issue with more than one corken at this point. Whenever i talk to either ETS or Corken, they tell me that the bleed back you are experiencing is normal because the seals that those check valves sit on are metal on metal contacts. Im guessing that the small amount of “Guck” you pulled out of it was actually helping you create a seal.
One thing to make sure of is that when you’re replacing those check valves you need to crank them back onto the Corken pretty damn hard to try and pinch that metal washer as much as possible.
Also, after blowing out your back line, applying vac to the front side of the system up until the inlet check valve should “Prime” that check valve according to ETS and Corken.