One other thing to consider. The 1 way flow valves (not sure of the "correct" name) should be checked for backflow.
I noticed that it was taking longer and longer to fill my ballast, and subsequently, I was frying my impellers. (2002 X-Star)
It dawned on me to plug the water exit holes one day, and wham, the ballast started filling faster.
I found out that all 3 of the plastic 1 way valves were shot, and letting air back into the system.
After a $30 trip to Lowes, I had them replaced and the system is like new.