I have been stalking the site and thought I would hit this forum up. I have seen several posts over a time frame of 6 years going over my exact same problem.
I have a 1.5" Waterway round filter, 22 inch with 150 pounds of sand (recommended amount of sand). The sand is one season old.
In a nutshell, I put a sock (regular old sock) over the return port and found that large debris is coming out. Not just dirt but big pieces of bugs, pieces of leaves, husks from seeds and blades of grass. My spider gasket looked worn but nothing leaking out the backwash port but I changed it anyway. No love. Still happening. I checked the laterals. They are fine. I am not getting sand in the pool so that would rule it out to. Like others from this site, I think I have narrowed the problem down to the riser pipe that feeds in to the MP valve. The water must be bypassing the sand through the connection of the riser pipe and the bottom of the MP valve. They just meet together loosely. No gasket at all. I have looked on sites for exploded diagrams and none show a gasket that I may have overlooked.
This is a conundrum that I would hope people can add a bit of advice. What do I put on the riser pipe to make the fitting more sealed, forcing the water to go through the sand and not bypass it?
Thank you in advance for any help or insight you can provide me!