To the left is an original CJ-3A gas cap. There is a lot more going on inside these caps than meets the eye. Original CJ gas caps have two valves inside. The first is a low pressure valve to reduce spillage when driving on side hills. This valve allows a slight amount of pressure to form inside the tank before releasing. Normally the valve is closed, so any fuel that splashes up inside the cap does not spill out, as it would if there was just an open vent. The other valve allows air into the tank as fuel is removed by the engine, thereby preventing a vacuum from forming inside the tank.

Specifications from the 1948 Mechanics Manual indicate the pressure valve opens at 1-1/2 to 2-1/2 psi and the vacuum valve opens at 3/4 psi.

Many replacement caps do not have any valves and are simply open to atmosphere.

Here is an inside view of an original type gas cap. There were a couple different designs with minor variations so your cap may not appear exactly the same on the inside. The four slots around the perimeter are open to outside air, and the center hole is open to the tank interior. Some original caps have two interior holes located just inboard of the neck seal, instead of one hole in the center.

Here is a cutaway cap showing the two check valves. In the very center is the small vacuum check valve. There is a lightweight spring (just visible) holding the valve in the upward position. When a vacuum begins to form inside the fuel tank the valve will unseat and allow air to flow into the tank. The larger spring is holding the pressure check valve down on it's seat. Excessive fuel tank pressure causes the pressure valve to unseat, moving upward, and releasing pressure out to atmosphere through the vent slots.

Here are the components laid out side by side. The pressure valve seat can be seen in the center of the gas cap body. Between the pressure valve spring and the cap body is the pressure valve, with the vacuum valve built into its center.