I thought I was talking to other yank A&Ps. I am not familiar with the British Commonwealth Aviation Regs or even if you have any. those publications are FAA publications.
I just snapped that everybody is an engineer, anyway I looked in the FARs ( Federal Aviation Regulations CFR14 FAR 121.383 ( Operating requirements: Domestic, flag and supplemental operations ) pertaining to air carriers and of course only certificated airmen with the privileges allowed under that ticket ( rating ) do maintenance including preventive maintenance, but if the TSA here will let them on the ramp, they are cleared, and if the Operator wants other people doing walk around inspections well that's their business. and one more pair of eyes isn't going to hurt, just as long as any discrepancies are brought to the attention of the People responsible for Logging anything if necessary and completing the requirements of whatever progressive inspection program is in effect at time
Heres something I found FAR 37.167.4.7 : Leakage: After an initial 12-hour minimum stabilization period, the tire must be
capable of retaining air pressure with a loss of pressure not exceeding 5 percent in 24
hours from the initial pressure equal to the rated inflation pressure I read through a lot and there is no mention of the valve or the cap, so it's manufacturers specifications and I am not gonna open a A 320 publication no way.