When bolts are overtightened, it can cause rubber gaskets/washers to pucker which causes 'em to leak. That you're tightening 'em so much that you crack the base tells me there's a good chance that's what's happening.

I see this a lot on head discharge fittings. The flange on a joker valve is also the gasket that seals the discharge fitting to the pump housing. People become over-zealous in their efforts to make sure that connection can't leak...and cause it to leak. And sometimes crack the pump housing.