As promised, I dug out my Soo Line Air Brake Rules book, but unfortunately, it slightly post dates the steam era: December, 1962. However, here are the pressures specified at that time, all numbers are PSI:

AIR COMPRESSOR GOVERNOR -130-140
FEED VALVES, PASSENGER SERVICE - 90
FEED VALVES, ALL OTHER CLASSES OF SERVICE EXCEPT ORE SERVICE - 75
FEED VALVES, ORE SERVICE - 90
FEED VALVES, SWITCHING SERVICE - 70

As one can see, the pressure setting was variable with the service, and as the feed valve was in th cab, the engineer was expected to ensure it was set properly for the service.

Dennis Storzek