One of the most common reasons for a flat tire is a leaking valve stem core. A valve stem core is a small device that can be threaded and unthreaded out of the valve stem. This tiny core allows the tire to be inflated but restricts air from coming back out. Tiny pieces of debris can enter the core causing it to malfunction. This debris causes the core to stick and may result in air leakage. Tire centers use a special valve stem core-pulling-tool to replace and remove this device.
Remove the valve stem cap by rotating it counter-clockwise. Insert the valve stem core-pulling-tool and twist it counter-clockwise until the core is removed from the stem. Throw the old core away.
Insert a new valve stem core into the valve stem with your free hand. Push it down into the stem with the valve stem core-pulling-tool; twist the tool clockwise to thread the core into the stem.
Thread the core until the air flow coming out of the valve stem stops. Fill the tire with air to ensure the valve stem isn't leaking. Squirt some water into the valve stem to look for air leaks in the new core. Blow out the water with an air hose and replace the valve stem cap.
