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Bridgestone/Firestone engineers recommend using double-seal, flow-through valve caps on all tires. These contain an internal check valve, similar to the one in the stem. When you apply the air hose, pressure gauge or air chuck to the valve cap, the air flows through the cap and its internal valve then, through the valve stem into the tire. The cap has a ring-like seal that seals it, like a conventional valve cap, to the valve stem. Inside the cap is a second seal that keeps the valve inside the cap closed until you apply an air hose or gauge. This provides a positive seal.