Capt Larry. On your 450 you will find a one way valve located close to the lift
pump. It will tend to collect weeds and other debris preventing it from closing and will drain the water
above it. Remove, inspect and clean it. Its now part of your regular maintenance
We also replaced this OW valve for a better quality one however any valve will malfunction
if there is a small twig in it.
The run-dry impellor is also a good solution.