A/C vacuum loss
 

A/C air shifts from center vents to defrost when manifold vacuum is low (WOT). Just started doing it, is there a check valve or reservoir that is common to fail? It just happened and I have not had the chance to look in to it. Thought I would ask this forum for help. Any ideas would be appreciated. Thanks

Get a MitiVac hand pump and attach to the supply line in the engine compartment. If the system won't hold vac, you need to fix the leak.

If it does, then you have low vac for another reason, and will have to find out why. Old hard rubber connectors are a typical problem -- they leak, and you have low vac in that line.

Peter

Vacum Loss
 

The locking pump in the 140 shows a connection to the HVAVC, I dont how how it does it or what triggers it, I would check hoses or check valves

I had this exact problem on another car (not MB), and it was caused by a faulty vacuum check valve.