Lil-Lynx, I have just had the exact same problem. After 2 years of seemless operation, my EX1200 decided to leak and dump 10L of water all over my carpet yesterday. Nice! After firing off a volley of colourful adjectives, I settled down (with rapidly soaking knees) to figure out what had happened. I replaced the O ring and smeared it with a little vaseline and put the filter back together. It ran for 10 minutes before leaking again. On taking it apart again, I noticed that the O ring has two mould lines on it where the rubber is uneven. When the o ring is sitting naturally (ie. untwisted) These mould lines have a tendancy to align themselves so that they sit against the sealing surfaces of the filter. End result, leak! So before putting it back together again (cue more colourful language!) I rotated the O ring so that the mould lines don't come into contact with filter head or body. Also, as insurance, I ran a layer of insulating tape around top of the inside of the filter body (the width of the lip) taking care to butt the two ends of the tape together (rather than overlap them). The filter has now been running for 24 hours without a leak, so it seems to be working in the short term. What I'm going to do in the long run is another matter. Coming home to a half empty tank and floor dripping with water has kind of knocked my confidence in the filter! I may change mine, which is a shame because apart from its rubbish carrying handle, I liked the EX1200.