Originally Posted by spannerhead
This pdf from a manufacturer implies (p49 I think) that the cap is solely to keep dirt out. Surprised the hell out of me because I have always believed that the cap was the primary seal.
If the cap was the primary seal the gas would all escape when you removed it. As you say, and correctly found from the OEM, it is a secondary seal whose main purpose is actually to keep dirt out not gas in.
Re MEL I was always taught if it's an airworthiness item and it's not in the MEL you don't fly if it's not there and working. Many interior items fall into the 'non airworthiness' classification hence HIL items, and I would interpret valve caps the same way.
I've never known an engineer who didn't have a bag full of them anyway, or at least a full set on his car
