The two common types of valves used on bicycle inner tubes are Schraeder and Presta.
Schraeder valves work just like the ones on a car. You can depress a center pin to release air. The pin is depressed by an inflator nozzle when you add air.
With a special small forked tool, called a valve core tool or valve core remover, the entire Schraeder valve can be removed from the inner tube and replaced. There are long and short versions, and are entirely interchaneable with automotive valves.
Valve core tool with short Schraeder valve cores
Available from Grainger
Presta valves are a bit lighter, and smaller in diameter. Some bicycle rims are too narrow to accommodate Schraeder valves. Presta valves are as fragile as they look. To add or remove air, you must first loosen a small nut at the tip. This is non-removable. You only loosen it enough to allow the inner pin of the valve to move up and down. Then you can press the pin to remove air, or use an inflator to add air. Unlike Schraeder valves, the pin is not spring-loaded. It depends on air pressure to keep the valve closed. To augment air pressure locking the valve, the nut can be secured.
People have often broken these nuts off. The good news is that the valves tend to work OK without a nut.
A third type of valve, the Woods valve, also known as Dunlop valve, is rarely found on older bikes in Japan, and European and some third-world countries. This is the same diameter as a Schraeder valve, usually has a removable core, and can be filled with a Presta valve inflator.
Woods or Dunlop valve
Photo by SCEhardt
Most valves have optional valve caps. The purpose is to keep dirt out of the valve for proper functionality. Even without the caps, most valves provide many years of reliable service.
Do you have a slow leak? Most slow leaks are punctures in the inner tube, but some are due to a leaky valve. With a Schraeder or Woods valve, you can put some water or saliva on a fingertip, hold it lightly over the top of the valve, and look for bubbles. With a Presta valve, you can use a similar technique, using a cloth and some soapy water. Leaky Schraeder valves are often just loose. Just give them a twist with a valve core tool, and you’re all set.