My '89 190E has a peculiar problem. The vacuum activated valve (can't think of the name of it - located just below the windshield [and a plastic screen] top, center on the engine side of the firewall) that shuts off engine coolant flow to the cabin heater, works only in cold weather.

I can turn off the heat in the cabin when it is really cold outside, but when the weather warms up, there is no vacuum to the valve (the valve itself is new and tested fine). The vacuum line to the valve has no vacuum at all (I've only tested it in warm weather, when the AC was losing ground to the constant flow of heat in the cabin).

What controls this vacuum, and what would the likely cause of temperature-dependent failure be?

Thanks.
-Brett