190D heater

[WANT '71 280SEL]

My brother's 190 has had a wierd heater for the past year, but I'm just now getting ready to look into it. When you use the defroster, the air will blow cold, but if you turn it off then back on in a couple minutes, it might blow hot like it should. Or, if it is blowing warm, it will just instantly go to cold until switched off, waiting awhile, then back on. And even that doesn't always do the trick. I was thinking maybe the heater valve that's vacuum controlled? Where is this valve located? Do you guys have any ideas as to how much vacuum I should be pulling there?

His car is the epitome of a vacuum system gone bad. Sometimes it won't shut of correctly, door locks just whine, center vents freeze you out if you leave them physically open in the winter. Hey, starts the best in the winter though! (my car has had 1 GP out since I've had it, so I don't know how well it could start yet.)

Thanks
David