Valve Cap Missing
Just checked CAAIPS, and was unable to find any info that says the Valve Cap is the "Primary Seal". Checked the Michelin Tyre website.au and It states: "The valve cap is the primary air seal and must always be fitted ", but this was for Truck Tyres. I've always been taught that the valve cap is primary, and must be there. Having said that, the AMM for B737NG for my company mentions removal of the valve cap for servicing of the tyres, but neglects to tell you to replace the valve cap on completion. ( I think that's a failing on Boeings part), however, common sense and engineering practice should ensure it's refitment at the end of servicing.
As there is no mention in the MEL, AMM or DDG/CDL for a valve cap allowed to be missing for flight, a straight forward ADD should not be adequate for despatch. I reckon an NTO from your company's Quality department and/or Aircraft manufacturer should be obtained.
Experience has shown though, that the valve cap WILL NOT prevent a leaking tyre inflation valve from deflating the tyre over a period of a few hours, and only recently had to change a wheel for this on a B777. And yes the valve cap was found to be tight on arrival, so it's ability to be the "Primary Seal" is flawed.
I may be new to posting here, but been in the industry 30 years (Licensed in engineering, not security)