Does the air pressure cap on my tires keep air in at all?

No - it’s not a “pressure cap” - it’s just there to keep dust and dirt out of the “schrader valve” which is what actually keeps the air in.

If you remove one of those dust caps - and you’ll see that your tire stays inflated - and no air is rushing out of the valve stem.

If you look inside the valve stem, you’ll see a small metal gizmo sticking out of the stem - pushing firmly on that would cause air to release.

When you fill up your tire, the tire pump pushes down on the schrader valve to open a path for air to get into the tire.

However, if you don’t have caps on your tire valves - then dirt and water will very soon get into the valve - which will eventually cause a loss of pressure. Don’t do that - you need the schrader valves to stay clean.

Schrader valves are used in many other places in a car - notably, the air conditioner coolant is kept inside the A/C unit by a pair of schrader valves - each with their own dust cap.

I’ve seen them used in fuel sensor monitoring points as well as in power steering fluid test ports.

In each case, the dust caps are important to protect the valve - but have no effect on maintaining pressure.

Source: Quora.com