I assume it's a close coupled cistern [direct on the back of the pan] if so then obviously turn off the water supply then flush it. Then bale out the remaining water. Spray securing nuts with WD40 to ensure ease of removal. The look at the back top part of the cistern and if there are securing screws into the wall remove them. Then undo the nuts and ease the cistern off the toilet pan. Between the pan and the cistern there should be a doughnut shaped seal, check it's position and re-serviceablity. or just take it to you plumbing shop and get a new seal. Don't forget to paint your wall before re-assembly.
Bob
Bob
September 2005
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