A possible reason that wasn't mentioned about why the o-ring leaks sometimes is that the 4 screw hole arms on the cover touch the face of the pump before the rest of the cover can come down all the way to compress the o-ring. At least that's how it is on my Mag5, it worked for over a year before developing an extremely slow leak. Maybe the reason that only some report no problems is that the degree to which the screw hole arms extend below the face of the cover has some variance.
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