Funny that this post should appear when it did. I just spent this morning solving this very problem.

Lubed the gasket on the pump where you access the basket. - bubbles
Checked the tightness of all the fitting on the plumping coming into the pump - bubbles
Replaced the Teflon tape on the fittings to and from the pump - no bubbles

I think the real cause was the intake fitting wasn't tight. I think over time the installing and removing of the pump each winter loosened it.