Do you have a gauge or way to spin the pump cam on the bench & power/pressure it up?
Pressure filter gummed up or unloader valve scratched or o-rings on that shrunken/leaking?
The pump seems to last forever.
Sometimes if the bores are egged out in the raw aluminum after 500,000 miles or so, they?ll leak same as the raw aluminum directional valve housing on the (ZF) otherwise quality hydraulic-assist PS rack & updated pistons from the Teflon pistons w/fresh o-rings won?t fix it
Swap the valve body or Teflon coated steel insert sleeve it. bTDT.
At least that *is* fixable unlike a shattered cast aluminum cone clutch hub, blown overrunning clutch or shattered hollow gun-drilled main shaft (plugged pressurized oil passage on the main shaft between the valve body & overrunning clutch is a good riddle to diagnose).
Cone clutch or its thrust bearing it won?t decelerate or reverse/slips in reverse, usually?
Lots of parts NLA, gear vendors (who bought GKN & all their tooling/inventory & that was Leycock before that iirc?) wants the whole unit with their tag on it most times to do anything/won?t sell individual parts & wants $4K+ to do anything for a whole new or reconditioned unit & has made most parts proprietary/incompatible with the Volvo-application OD?
You used to be able to buy a new OD (or new guts / reconditioned tested unit complete, anyway?) from Volvo in their exchange for $800-$1000 (whatever that is now, inflation-adjusted in 2022 dollars instead of early 2000s dollars?) or so?& still can iirc??outta most Volvo guys budget or they?d run an M47, T5, UK takeoff low mile M90 or M51 getrag, instead, at that price?
This said, direct drive or planetary gear set for OD worked good for towing and no thrust load in 4th & OD to wear out the transmission bearings/other parts, so I do like the direct drive in final gear (4th in most cases or 1:1 5th with the M51) w/OD as a configuration, if not the POS British OD unit in execution/quality control, so much?
If it engages in torque multiplied slow reverse up a hill or speshl thrust-rated ball- bearing or cone clutch is marginal, it?s often game over / full of hardened metal paste? not much worth saving/can be reused except the alum housings.