300e Center Vent Blows Then Closes Down

1988 300E - center dash vent used to blow continuously on a/c and cool air economy cycles - now two minutes after start up, air comes from two side vents only - replaced button & temp. control unit - not cured - suggestions?

[vinamg]

You have a leak in your vacuum element. Remove the glove compartment and you'll see the defroster vacuum element on the left side. Check if it's leaking.

Vin

[J.HIDALGO]

does not work. Probably is another vacuum servo. There are 6 or 7 of them under the dash board. Unfortunately, there is only one or two accessible when you remove the glove compartment. For the others, you have to remove the dashboard. A BIG job for you or BIG $$ if somebody else does it for you. Most people replace those servos as they replace the evaporator since they have to remove the dash board anyways.
One thing you can do is check the green? vacuum line(s) entering the firewall from the engine on the drivers side, sometimes they get disconnected.
If you can live with it, just leave it alone. If not, well...good luck!

[Gilly]

It could be normal. The center vent, as you stated, blows only cool air. If the system switches to heat mode (which is dependent on the temp wheel setting and outside temp) the center vent will shut off. When this happens, is the air coming out of the side vents heated at all, or cold air?
Gilly

happens on cold fresh air and a/c - I'll be back after checking vacuum in and pulling glove box and report on findings