A common fault with the CAV is people put the wrong 'O' ring in the wrong places. The 2 that fit above and below the element are NOT the same size and this is the usual place people stuff up.
Apart from that it is always possible the top housing is warped. Being new, I'd be inclined to take it back.

The through bolt can be done up fairly tight. The thread will bottom out so over tightening it should not be possible.