You step out into driveway ready to start your morning commute only to discover a flat Tire. How in the heck did that happen overnight? If the tire valve is missing its cap, the culprit might be a leaky valve. Those little caps keep out dirt and moisture that can cause leaks, so be sure to keep caps on all your tire valves. Another tip: When you replace tires, remind the tire shop that you expect new valves with the tires.