My old Ford 4100 tractor has a diverter valve on the top cover for loader hydraulics, the hydraulic power for the 3-point linkage passes through the interface between the valve and the top cover. When I have a heavy-ish implement on the 3-point linkage, hydraulic oil leaks out from between the valve and the cover. All the O-rings were renewed about a year ago when I did a heap of work on the tractor, fixed all the other oil leaks but this one persists. The two mounting surfaces were a long way from flat, so I've used the surface grinder to make them flatter than a dunnyman's hat, now I need to clean up the surfaces in the bottom of the O-ring recesses to give the new O-rings a fair chance of sealing.
diverter valve surface with bad seats.jpg

There are 9 recesses in 6 different sizes. The valve body is cast iron. My plan is to turn a piece of stock to match each recess diameter, mount the valve in the mill and use some lapping paste on the face / end of the round tools to polish the bottom of each recess with light pressure on the quill. My main question is am I best using steel, aluminium, hardwood, softwood or something else for the polishing tool? A second question would be any other suggestion to make sure these O-rings don't leak again.
And yes the mounting lugs are broken but I have already sorted a method of clamping it down tight so that is not a worry.
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