Got a IH truck engine problem 

This engine always used to run 20psi at the lowest at idle. Now on both a mechanical and electric guage it goes as low as 10psi. The dash warning light will flicker at that point.

Looking in a IT manual for IH tractors it shows 2 different valves in the engine. One they call a "pressure regulating valve" and the other they call the "bypass valve". From what little IT manuals say it seems like the bypass valve would be the thing to look at since it is in the filter mount but what exactly does the pressure regulating valve do on this engine?

Redwrench or anyone?