How important are Tire Valve Stem Caps in retaining tire air pressure?

I was wondering if anyone knew how important tire valve stem caps were in maintaining inflation pressure in auto tires? I noticed one of my tires losing air slowly (from 30 psi to 21 psi in about a week) and this tire (unlike the other three) also had a missing tire valve stem cap. I've just replaced it, and... show more I was wondering if anyone knew how important tire valve stem caps were in maintaining inflation pressure in auto tires? I noticed one of my tires losing air slowly (from 30 psi to 21 psi in about a week) and this tire (unlike the other three) also had a missing tire valve stem cap. I've just replaced it, and was wondering if a missing valve stem cap caused faster loss of tire pressure.
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