The problem with that type of valve is the stem, I have a similar valve on my R410A manifold.
Turn a valve and watch the vacuum pressure come up.. They leak!!
I hate leaks!!
Maybe they were designed to hold pressure inside, and not work the other way.?.
With a vac check valve, you could have them installed right on the panel's port, in areas that weren't normally accessible.
Just suck the air out of the attached hose, and the rest is automatic.
If the check valves were cheap, you could stick some in the lines going to the
VacWall's-manifold(?), so if there was valve leakage, the feed line would already be at a low pressure,
and slow the leakage down.
Have you ever seen these things?? Very few of them hold low pressure very long..
