Water leak - Purex/Triton Valve issue - Help

Help! Over the years I have had repeated issues with water leaking past the "spider" gasket while the filter is running and water going out the waste outlet. I have replaced it about 3 - 4 times in the last 10-15 years (30 year old filter)

The gasket doesn't stay fully seated in the channel of the valve. It pulls slightly over and allows water by, into the outlet/waste chamber.

I have tried replacing the gasket with and without adhesive to hold it in place. I have even tried using a plumbers grease on the top face of the gasket. The last time (about 9 mos ago) I cleaned everything, removed all the water in the valve, dried it thoroughly. Replaced all the o-rings and gaskets, even the nylon washer that the handle pivots on.

It seemed to have held up fine till today. I try to be careful when I operate the handle. I push it fully down and rotate. Then I allow the handle to rise back up, trying not to push any "lateral" force on the edge of the spider gasket.

Anyone have an idea what is wrong? All the parts appear fine. No wear, no cracks, the valve is smooth. I had thought about making a slight radius on the edge that contacts the gasket but it doesn't seem that I should have to. Are the newer valves (the part that rotates) any different than the old ones??

I don't know the answer to your question, except to say that I replace the spider gasket in my multiport about once per season. I just figured mine gets more wear and tear, because my pool is open year-round, but that may or may not have anything to do with it. My thinking is that what little cold weather we get probably contributes to it drying out and cracking, but I dunno.

I, like you, have tried both with and without adhesive, and it doesn't really seem to make a difference--but for $15 gasket and a 10-minute job, I just replace it and move on.....

Ya know, I've never had to replace mine. Ever. Wonder why some need replaced and others don't?