I just measured the vacuum to the heater valve (at the front of the engine compartment) and was rather surprised- I had thought that a vacuum at the heater valve would OPEN the valve and allow coolant to flow to the heater core. Now that I've measured it, I found that this valve only gets vacuum when the selector is at "MAX" or "OFF". I guess that would make it the exact opposite; the heater core always gets hot water unless a vacuum is present at the hot water valve. Just thought I'd verify this through you guys... Jim 1537