[Tom Endy]

What you have is a standard poor quality repo gas cap that is prone to plugging of the vent hole. The small hole above the single rivet is the vent hole. That hole goes through three things, the metal plate that grabs the tank boss, the red fiber washer, and a brass wafer that is under the red washer. All three of the holes must stay in alignment in order to vent the tank. What happens is the center rivet loosens up with the on & off action of the cap. The red washer drags on the tank boss and all three items with the holes rotate out of position and the tank no longer vents. The whoosh sound you hear when you remove the cap is air rushing into the tank.

There is a way to modify the cap. Drill the existing hole out very carefully so as not to go through the roof of the cap. That will restore the vent hole. Drill a second hole 180 degrees from the first hole. Drill for a tap size of a 6-32 screw. Make sure you drill through all three items, but don't go through the roof of the cap. Tap the hole and screw in a 6-32 screw. Coat the screw with either lock-tight or JB weld. This will prevent the screw from falling out into the tank and will prevent the three items from rotating on the center rivet and keep the three vent holes in alignment.

An alternative is to order an Eaton cap from Snyder. The cap has two rivets and two vent holes and though not identical to an original cap, it is close, and the vent holes will stay in alignment.

Tom Endy