[Thekrents15]

Jeff,
It is a normally closed contact set.

emergency brake operation:

emergency brake set - no pressure on the output side of the valve.
Hobbs contact normally closed - anything watching that signal knows the parking brake is set
emergency brake released - output side of the valve pressurizes
hobbs switch pressurizes and contacts open - anything watching that signal knows the parking brake is released

My refrigerator lock/unlock is following this switch. With the Hobbs switch removed causing open contact to be seen (pressurized, parking brake released)
the lock/unlock does nothing.

Seems to be a very low threshold, 4 psi, to declare parking brake released.

That is the way I see this is working.

Thank you all for helping me work through ... another air leak