300E Air Conditioning

I am troubleshooting a center vent vacuum element issue on a 87 300E.
I get vacuum on the line, but apparently now enough vacuum to open vents
If I bypass the switch-over valve(124 800 0178) and connect the center
element to engine vacuum line(found on the switch-over valve inlet), the vents open.
the switch-over value is $105 new and I would hate to replace if the issue is somewhere else.
Mercedes Benz don't stock this item, so I was wondering if this is a common issue or not.
Do they get clogged? what is the average life? Should I buy a used one from salvage yard?
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Have you replaced the yellow checkvalves yet? This is the first thing I would change as it does go bad over time.
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do you mean the gold metal inserts inside each port on switch-over valve?

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No... I meant the yellow thing in the hood area by your fusebox where it has one hose connected on one side and two hoses on the other side.
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