Any idea where I can get the high side service cap port for an old E-150 van? Would prefer not to go to a dealer, local parts store, didn't have it local dealer had no listing , its an odd ball size from an R12 system. I had the van in for service and the cap mysteriously vanished!

I have a plastic cover on it to keep the dirt out.

Will not having the cap on cause a leak, or is it just a dust cap to protect the Schrader Valve?

Thanks