It could be that the faulty gas cap is vented in only one direction, allowing air into the tank but not out of it.
A hot day with only a 1/4 tank of gas (3/4 of a tank of air) with no way to get out will likely cause some problems (like forcing fuel past the needle/seat).
I do not know how the vents in these gas caps are supposed to function. Do they allow air both in and out.
Carl
