I have parts and information on stock here for the Gemmer Power Steering head. This head was also used on the later 770/880, 1800-A Series Oliver tractors and ALL the COCKSHUTT built 550/560/570/570S tractors with the P.S. option.
I do not know anything about the power steering pump or hydraulic pump on your 1800 Oliver.
If you think your problem is in the PS head, I can assist you.