I have had this issue for awhile now. My Pentair canister filter leaks where the lid mates with the bottom portion (see circled area in pic). I've tried a new o-ring, lots of lube, a little lube, nothing would seem to get a good seal consistently. I could usually get it to seal after about 10 or so tries, but it seemed like pure luck.
I eventually decided that the lid might be bad (warped or cracked or something), so I bought a new one. The first time I installed the new lid, it sealed perfectly. I thought, awesome, it's fixed. Well, I must have just got lucky b/c the next time I cleaned the filter, I had the same leaking issues in the same area of the filter.
So, I need some of the expert advice. The pool was built in 2005 so I'm guessing the equipment is the same age. I bought the house in 2010, so I can't be 100% sure on that. Is it just time to replace this thing? Should I try a new o-ring first? This has been an issue for awhile now, so the o-ring is probably over a year old now and has been removed and replaced several times. I'm not too sure how resilient those are, but as far as o-rings go, it still seems good to me. Any other ideas I could try?