Sometimes when new bearings get installed in the cover plate, they're not seated quite deep enough and that holds the lens/valve plate off the cover just a bit, same for locator pins. Or there is a burr or bump on the cover or lens plate.
Just to eliminate some simple things, swap in relief valves from a working unit. Just the charge pump going over relief should haul your drill down.
Since you have to pull the hydro anyway, might as well open it up one more time. I've rebuilt hundreds of them in 28 years in the shop, but never "online".
Might be fun to do some virtual hydraulic sleuthing.
And I believe you're correct about the pinion.
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