They may seem like small things, but the valve stems are very important. The first thing to look for is the presence of a valve cap. They may be small, but valve caps perform a vital function in keeping the valves clean and preventing slow leaks. Even a small amount of soil in the valve can lead to a slow leak, which can then mean a persistent under-inflation problem. Not only should the valve stems be checked regularly for leaks, but it is also important to replace any missing valve caps as soon as their absence is noticed.