I've got vacuum at the check valve, but not at the selector switch.
I unwound the wire loom that the black vac line is in. It's fine as far as it goes into the heater box. So I'm thinking its leaking inside the cab. Can not hear any leaks under the dash.

Any idea's on where the problem could be?

 

Have you checked under the hood above the fan unit for cracked/ brittle vac lines? Sometimes even just past the fan box it can be leaking from the inside