I would say that there is a seal that has a challenge..
If you go through the book, you will find a page that shows the spacers, seals, piston and the body along with other parts.
By putting it in bypass and starting a cycle to releave the pressure, the powerhead will come off and the plate under it and then the piston and seals and spacers come out... now for now you might try changing the line up of the seals ... ie taking the middle ones and moving them to the top and bottom .. Look them over at the same time to see if there are any breaks... or cuts...
If the piston or seals and spacers need to be replaced there are is few spots on the web that the rebuild kits can be had and most likely under 60.00 and then your time to put it back together.
What kind of water quality do you have?
Almost sounds like some one trying to make a sale of equipment...but that is just me.