Not sure if it's just my computer or what, but I can't get any IH parts books to open lately. Though not identical, most IH tractors of that vintage have similar systems. Relief valve should be somewhere in the cover plate they call the MCV. That should cover both the lift and auxiliaries. Is the problem limited to loader response or is the three point also weak? If the three point will lift a load, then the relief is probably not an issue. Is the loader supplied from/through the remotes? Steering and PTO working okay?