How often should the rear wheel bearings be lubricated
on a PV544? (this is much more complicated and expensive
than lubricating the front wheel bearings)

Volvo recommended repacking the rear axle bearings every 25,000 miles.

However, I have found that if the bearings are repacked every time the rear brakes are done, the bearings will be fine.

I also like to replace the inner axle seals at the same time. If you don't and an inner axle seal fails, the gear oil will wash the grease off and the bearings will quickly get damaged.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502

Thanks Planetman, my mechanic says the cost is around $700, but must be
necessary.

$700? Too much.