· Don''t think you need a 3/8 vent, my roadster (350 Chev) came that way, same problem. Plumbed w/ areoquip lines. My fix was a 3/8 dia. alum piece, 1" long, drilled a 1/8 dia hole thru, put in end of hose, clamped it, problem solved for me. The vent line needs to be above top of tank, added check valve at tank.