A valve sandwiches the rim between two sections of rubber on the valve to create the seal.
Most tire leaks come from damage due to a road hazard, such as a nail. Those leaks are readily identified. Often, cause of unidentified leaks is your valve stem. The valve stem creates a seal, using the rubber base of the valve stem that presses against the rim. The base of the valve stem, when fully sealed, should be ample to maintain the seal, but a damaged hole on the rim will cause the valve stem to fail to create a seal. Sealing the valve stem is simple.
1. Remove the tire from the vehicle. Raise the wheel with a jack, and remove the lug nuts with a lug wrench. Set the lug nuts to the side, and place the tire on a flat surface.
2. Release the air pressure in the tire by removing the valve core.
3. Break the seal between the bead and rim by placing a pry bar between the rim and bead and applying substantial downward pressure. Push the bead down, until you have sufficient access to the inside of the rim, where the valve stem is located.
4. Remove the valve stem by screwing a valve stem mounting tool onto it and tapping the tool with a rubber mallet, until the valve stem is forced back through the hole.
5. Apply a small amount of bead seal to the hole, and reinstall the valve stem in the reverse order of how you removed it. You can, instead, install bolt-on valve stems by inserting the bolt-on valve through the hole and bolting the unit from inside the rim. Tighten both retaining bolts, using a wrench on the inside of the rim, until the bolt-on valve stem is secure. Bolt-on valve stems provide a much stronger seal than bead seal can create. However, both options will allow the valve stem to fully seal.
6. Stand the tire up, and lean on it, to create some inward pressure. Slowly air the tire, making sure the bead seals all the way around. Reinstall the valve core, and raise the tire pressure to factory recommendations. Spray some window cleaner around the valve stem to ensure it is no longer leaking.
7. Install the tire onto the vehicle, and tighten the lug nuts. Slowly lower the vehicle to the ground. Tighten the lug nuts to factory recommendations with the lug wrench.