Dorman makes a couple of fuel line check valves for 5/16 and 3/8 fuel line. I put one on the Edelbrock 625 I was running a while back and it totally fixed the problem.
Dorman 800-195 is the 5/16 and 800-196 is the 3/8. They're kinda proud of them at around 30 bucks each, but that fixed mine.
