Iluv: Makes sense then. Approximately 6 (negative if you must....) psi would be proportionally equal 400 mbar as 14.7 psi is to 1000 mbar. And if anything less than 1K is vacuum, that also make sense. As for the problem with the system, it's that the system is drawing air in from outside the car around the cooling and heating "functions". So in the summer, when the car is at low rpm with the A/C on, the system draws warm outside air in and dilutes the cool air through the vents. And in the winter, and with the heater on, cool air from outside comes in through the vents diluting the warmer air. And this effect is most pronounced at low rpm. I can't think of anything else it would be since the effect is the same (in reverse, of course) in both scenarios.
Mike: OK. Sorry for what could be interepreted as the implied "positive" pump pressure thing. But that begs the question: What would you call negative psi? I dunno. I've used vacuum gauges on domestics for years and it's pretty much universally referred to as "psi" even when there's a negative draw. Guess I'll need to change my way of thinking if you have another term for it.