The fitting inside the sump often cracks away from the sump itself, causing a leak. This is normally because the bottom fitting has been tightened, and then the top of the tube has been pulled across to get the top mount fitted, leaving the whole thing in tension. You cant get all the way around the fitting inside the sump to weld it, so ISTR we used to thoroughly clean around it, and then smear the join with Araldite. Also sometimes the dipstick fitting would bottom out on the sump before clamping the tube, so we used to make up a small ring/gasket to go in first.