I know this thread is ancient but may help someone out in the future. I had the same problem of my Walbro 450 bleeding pressure off and taking a few cranks to start. Thought it was the check valve too. Bought a new 450 and checked the check valve this time by blowing as hard as I could into the port. Nothing, 100% sealed. Installed it and no change, still bled pressure off. Took everything apart and dicovered the plastic corrugated hose from the pump module to the fuel pump did not seal perfectly with the oetiker clamps, even though everything was snug. The corrugated plastic tube buckled slightly under the clamp enough to let air in. Swapped it out for some Gates rubber fuel hose from OReillys for a few bucks and snugged it with some stainless steel hose clamps and now it holds pressure like a rock after prime.

The problem is that the corrugated fuel hose is not elastic like the rubber fuel hose is, so in my case, it buckled enough to let air in.

 

Did you use 'SUBMERSIBLE' fuel hose ?