I don't know much about the C tractors, but none of my tractors (all IH) have adjustable oil pressure at the pump - only a spring loaded relief valve. Oil pump covers can get warped, blow out the gasket, or "gouged" by the gears, all of which can result in low oil pressure. The fix is to have the cover machined flat (or use a sheet of plate glass with some grinding compound and do it the hard way), replace the gasket, and be sure to measure the end play in the shaft and add another gasket if you need to to prevent further gear contact with the plate - remember to order the gaskets, don't just make one from whatever's handy, thickness is important here. The gear lash should also be measured and if too far out of spec they can be replaced. They are pricey, however, and in some cases it is almost as cheap to by a new oil pump as to replace both gears. Many people I know have replaced only the body gear (the one that isn't pinned to the shaft), and while not within spec, enough backlash was made up to restore respectable oil pressure. If you get no satisfaction with these fixes, then you should consider main, rod, and/or cam bearing problems. Again, I've never been inside a C, but if they are similar to A's, H's and M's these fixes will help.
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