A Schrader valve is a simple check valve found on every car tire. It consists of a hollow brass tube with an externally threaded stem. Inside the stem sits a spring-loaded valve core. When you press the small center pin, the valve opens to let air in or out. When you release the pin, the internal spring and air pressure push the seal closed, trapping the air inside.
Beyond tires, Schrader valves are used on fuel rails as diagnostic ports to test fuel pressure, and on air conditioning lines to service refrigerant. Always install a valve cap over the stem. The cap keeps dirt and water from entering the stem, which can stick the pin open or corrode the brass core, leading to slow leaks. Use a valve core tool to replace leaking cores.