Oops - one more thing. While looking at the old vacuum unit some more, I saw a second solenoid valve and remembered I added it for vacuum-hold, to prevent run-back through the pump. The pump has no valves, so in cycling applications, the vacuum on the inlet would pull air back and run the pump backwards after shutoff, loosing the vacuum. The solenoid acts as a check valve and disconnects the pump inlet from the receiver at shutoff. Any sort of passive check valve would have forward pressure drop (analogous to diode Vf), which would waste some of the vacuum potential. A little is lost though, due to the response time of the valve, but much less than the forward drop of a check would cause.
When you have at best, one atmosphere to work with, you can't afford much pressure drop in any part of the system.
Ed
