[Lynn Eades]

The relief valve shouldnt be bypassing, unless you have a blockage( or partial blockage) in the cooler circuit. Maybe the valve is in need of adjustment. Check the spring ball and seat. There is an adjustment proceedure in the U.C carrier book, which may well be the same.
Another angle; some a.c. type mechanical pumps are fitted with a neoprene type gasket that allows little more than the arm to poke through, at the mounting face. This would help by acting as a baffle.
Maybe you can add a drop tube to the return, so that the oil flow goes down past the pump, before it exits the tube.
Is the crank case ventilation at the sump clear? (if it uses that system)
Does it have a PCV valve and is it functioning?