is it low level / close couple toilet. if you isolate the water to the cistern empty the water depending on what toilet you have but the syphon has a 55mm plastic nut which you can undo with pump grips put the new one in tighten up fill the cistern back up
if you have a close couple you can change the doughnut washer as well is thats 10 yrs old that will be knackered. that one seals the cistern to the toilet
hi-spec plumbing, October 2006
