The function of the Chevrolet Gas Cap is more than just covering the inlet to the fuel tank where additional fuel can be placed inside of the vehicle. This is also the vent of the gas tank.
With the gas cap also being the vent, it is impossible for the gas tank to build up pressure that could cause a hazardous situation to occur on your Chevrolet. If pressure was permitted to build in the gas tank, it would not only place stress on the fuel pump causing it to prematurely wear but also the fuel lines in the fuel delivery system. A fuel leak is one of the most undesirable situations any vehicle owner wants to encounter.
One of the reasons for the vent to be placed in the Chevrolet gas cap is because of its position as compared to the fuel that is being stored in the vehicle. With the gas cap being at the end of a long tube that is connected to the tank, it is nearly impossible for fuel to leak out of the vent. Remember the vent is an opening to the outside atmosphere around the vehicle. If the vent was not on the gas cap and directly on the tank itself, fuel would make tit way out of the opening causing a hazardous condition on your Chevrolet.
The choices the owners have when it pertains to the type of gas cap they want on their vehicle include locking and free opening. With the locking covers on most gas inlet portals today vehicles have designed into them the external locking mechanism on a cap is not necessary anymore but can still be used if it is preferred by the Chevrolet owner.