As for your leak when filling...the filler tube plugs into an opening in the reservoir which is sealed (sort of) by a soft, thick, foam rubber O-ring. These invariably get hard with age and don't seal. When filling, stop when the reservoir tank is full, before the level rises up into the filler tube. You then have about 95% of the whole thing filled anyway.

If your reservoir leaks even when the level is below the filler neck, then suspect a cracked reservoir or a leak where the pump plugs in.

No experience with the 740 pumps, but I have disassembled non-working 240 pumps and found the impellers/gears totally grunged up with deposits and scale. A bath in vinegar (a weak acetic acid) and a minute or two with a toothbrush and they were working fine again.
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Bob: Son's XC70, my 83 240, 89 745 (V8) and XC60. Also '77 MGB and some old motorcycles