95 heater vacuum h2o valve flakey

In my 1995 Tahoe the vacuum valve that controls hot water to the heater core is acting weird. It wasn't working (mostly on), which results in hot water running to the heater core and thus hot air
inside the truck.
I'm not sure what's wrong with the small vacuum regulator, so I bypassed it by running the vacuum line directly to the valve, expecting that the valve would just stay shut. Well, mostly it does, but medium to high loads on the engine causes it to open, presumably due to low vacuum. I think I've determined that there aren't any serious vacuum leaks (tho maybe the temp control head is not happy), so I'm wondering if the small vacuum regulator acts as a check valve from the engine side?
As far as the vacuum regulator goes (this is the small cylindrical thing on the side of the AC can on the firewall (right side) -- it has a 2-wire connector and two vacuum hoses), if I unplug the connector and put a volt-meter accross the wires, there's no voltage either with temp controls set to AC or high heat.
There are no blown fuses, and the fan and duct directors/actuators work fine.
Any advice? Thanks! -Jaz
(Please excuse the 'burp' when replying)

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