If it only leaks when you flush then the donut seal is not doing the job properly . Take it apart again and make sure you have the large oval metal washer (which takes the bolts to clamp the cistern to the bowl) the right way round. Then fill the inside rim of the donut with Plumbers Mate or similar. Make sure on re-assembly that you lower the cistern squarely down onto the pan to avoid dislodging the donut .
Tighten the wing nuts evenly and firmly before securing the cistern to the wall
Cledlyn, July 2008
