I think your DEX-2400-K 0-ring may be shot

Unfortunately, Hayward stopped producing them in April of 2009. [I wrote a post on Trouble Free Pool about it here.]

Please check the o-ring for pinches and/ or squared off edges. For your unit, you may end up having to purchase the ~new S.S. replacement kit - but, it'll be less than a new filter.

** I've NEVER seen the S.S. replacement -- but I'll be happy to tell you what little i know.**