Tire valve caps pressure preservation ability has been up for debate for years. Generally speaking, the purpose for the valve cap is to serve as a cover for the tire valve. The valve caps prevent damage to the valve by blocking debris from entering into it. Most tire valve caps are plain plastic caps that screw onto the valve stem. There exists little help in actually preventing the escape of air if the valve fails. There are also metal caps, which have seals seated inside them. These seal around the valve’s mouth and seem to afford some protection against loss of pressure.
For more info, see How to Install a Tire Pressure Monitoring System