Hi all and thanks to Mercedessource for all the help getting "Old Yella" back to nearly original shape. My question concerns the heat in my old girl. I replaced the monovalve some time ago and it had the typical tear in the rubber portion of the valve. The replacement gave me heat, though intermitently. Now the heat will roast you but if I am on the road a while it will subside and eventually I will feel cool air coming out of the vents. Then the heat will come back and the cycle starts over again. Roast then cool. It is like there is a time lag as the heating system tries to maintian the desired heat setting on the wheel of the climate control unit. I know there is a little vent on the center of the dash and I am just guessing that air is introduced into this port either passively or by suction. The system then, if working correctly, compensates to keep the temperature in the cabin at a constant. Is my theory accurate on how this system is supposed to work? What are the possible troubleshooting areas? Restricted flow of cabin air through the little vent? If there is in fact a pump that draws the air in, could that be the culprit? And / or are there any other possible items I should be checking. Thanks in advance for any info provided. Mike G.