The seal has to be going out for the oil to leak. I'm pretty sure the seal isn't part of an integrated bearing and is just a conventional sacrificial seal such as the cam seal which relies on a *tiny* leak of oil to create a barrier against the greater volume... the problem is they're not long lived if there's debris in the oil, the pressure on the back side is too high or the shaft isn't rotating properly along it's axis (bad bearings or bad shaft). A bent pulley is another possibility to add to the list but that's generally included with a rebuilt unit.
Since the pump was once rebuilt and then actually replaced with another unit bent shafts or bad bearings in two different units are unlikely. The belt being too tight could also pull the shaft out of alignment but it would have to be virgin tight. That's why I suspect oil contamination or pressure.
I don't know why it didn't occur to me sooner.... Ranma... as in 1/2? Is that where your username came from?
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