hi, how i changed the o ring was to put lots and lots of rags around the bottom of the resovoir tank and removed the 3? i think bolts using a spanner and a socket.
i then lifted it quickly off the bracket and put my finger over the hole at the bottom.
i drained the tank and cleaned it up then replaced the o ring.
then replaced the tank and refilled, making sure there was no fluid lying about.
very simple and easy job.
you could always use a turkey baster or such to drain tank first before you remove it.
worked for me