OK, here is what that orfice does. It supplies fluid to that regulator piston along side of it, the one with an o-ring on it and a ball unloading valve below it. When your aux valves are in neutral, a passage hooked in parallel to the orfice is blocked. Also, the aux valves are actually closed center meaning no fluid flows through them in neutral. Well, the gear pump being constant volume is trying to pump that fluid some place so the fluid flows through the orfice, push's the piston down, which opens that ball unloading valve below it and the fluid from pump goes to reservoir. When you move an aux valve for use, that parallel passage to orfice is opened also, and it being much larger than the orfice, the piston moves up , which seats unloading valve ball and system goes on demand to perform useful function. By making that orfice larger you supply more fluid to the piston. If you make it too large, it will hold the piston down at all times and therefore you will have no pressure build up for work to be done. The original orfice was .024 but they found out .031 worked better on later models. As your valves wear, more fluid can escape past that blocked passage in aux valve and cause system to stay on demand even with valve in neutral. Soooooo, some time we drill the orfice a little larger to compensate for that. But I mean a little, like maybe .040 or .045. NOT get in there with a 1/16th inch drill bit.
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