1991 300CE climate control problem. Read for three hours but could not find a soloution in the posts here. It appears that my system is in default mode according to several posts, however could not find a specific answer.

A/C works, heat works, fan controls work.
Tested for leaks in vacuum with mityvac at Y7 switch-over valve unit and could find no leaks. Tested footwell pod and opens and closes as it should with no leakage with the mityvac.
Took CCU out of my working E420 to test if unit was OK. Appears fine.

Now the problem - heat and A/C all work but only come out of defrost and side vents. Nothing out of the lower/footwell vents and very little out of the middle vents.
Could this be the switch-over valve or the check valve? How do I further test these, or has anyone come across this before?
Any help is most appreciated.

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