Hi, I have a new hydramax. The water is either hot or cold - nothing in between. I've taken it apart to check that the cartridge works Ok and it does. it's a HP green one. Looking through other posts it seems that there might be a problem with the o ring but i can't find enough detail to check. The best I can do without draining the system again to get it out and photograph is a description :- The opposite end to the heat control of the cartridge is a 15mm (approx) pipe with a small conical filter in there. Around the outside of the that pipe is a rubber o ring. Should that o ring be fitted inside the plastic component of the shower enclosure. Looking onto the plastic coupling I can see (and feel) a small groove that might house the rubber o ring. If not then what is the o ring doing as when the pipe slides up and down in response to temperature control adjustments it seems to be about 10mm away from the hole where the water comes in. I don't think that the o ring is sealing anything. Any ideas or help would be appreciated. thanks pres.