If you can't pull vacuum on it at all, there's no way it'll open. Check how you did the test and visually confirm that the line you were pulling vacuum on actually goes where you think it does.

If it will open and stay open at "18" but works intermittently at "22" you have a pushbutton unit problem or an ambient temp sensor problem. Get a hold of the FSM and check to make sure that your vacuum lines are connected to the right places on the switchover valve unit. The switchover valve unit is also another common source of problems, there are some solder connections that can go flaky and cause erratic behavior.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]