does it have a charchol canister?
thay shut of tank breathing when the car isnt running.
vacume from manifold opens valve on canister, or valve in breather line.
if it has anti pollution gear it wont breath out of cap.
sounds like u have vacume in tank,its like trying to get fuel to flow up hill??
take off cap see if that works.
are the lines on the pump the right way around????

tommo