Importance of "dust caps" for schrader valves

[JohnHere]

I can't speak for GM Tech. As for the plastic caps withstanding 300+ PSI in MVAC systems, they are indeed intended to withstand such high pressures. In fact, the robust threaded design of the caps is sufficient for more than 400 PSI on the high side. Otherwise, most trinary (safety) switches wouldn't be factory-set to that high a limit. That's also why the caps have a rubber element or washer or seal in them, as GM Tech said.