I posted this in the Intairco trap thread...
Mike,
Regarding your MAP trap - I would suggest taking the cap off, remove teflon tape, lightly clean with acetone to get rid of any fuel residue and permanently hysol the cap on. Years ago I had similar issues with an old big can Jetcat P180 using BVM's UAT with enlarged fittings. Was running 3/16" lines, fittings, even dual vents but was still getting air in UAT. During some ground run-ups at high power I finally noticed air was getting thru the UAT cap threads causing the problem due to slight distortion when the nalgene UAT sides sucked in. Gluing the cap on worked, but I eventually switched to JMP's all aluminum unit because that was the only proven option back then. Now we have a wide range of trap manufacturers to choose from and most hard shell ones work perfectly fine.