After a few years the hold down arms and linkage can stretch allowing a bigger gap between the lid and canister allowing the O-ring to seat farther off its intended location and seap, something you may notice by looking at the the edges where the canister and lid come together if the gap is now larger when positive water pressure is applied (canister under tank primed not running). All you can do in that case if after you lock down the lid and push it all the way up against the canister, try and find a way to tie the lid in place using a good large plastic tie-tie top to bottom. Also having used a screwdriver or hard insterment to pry appart the lid from the canister body can cause nicks on the plastic edge where the O-ring seats allowing a seep. Look for nicks and sandpaper them out with 1000 grit wet/dry sandpaper.
