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.
Tire valve caps are the final seal again potential failure in the valve of the valve stem. A good tire valve cap will come with high quality rubber seal that push against the circumferential 'ring' of the valve stem and thereby help to contain the air should the valve fail or not containing the air very well.
You observation of your tire (the one with the missing cap) losing air confirms that this could be a possibility.
Of course, the other possibility is that your tire is having a slow leak – go to any petrol station, and it could be rectified at a minimal cost.
Note: Even if the valve is perfectly good, if the tire go over a bump or pot hole, it is still susceptible to lost of air due to the additional pressure asserted during the hit.