you'll probably find the seals failed on the flush valve.
hi-spec plumbing & heating (essex)
March 2010
Take lid off cistern and adjust water inlet mechanism to produce a lower level.
Operate the flush mechanism and observe the moving parts that shut the water off. Often just need to turn a nylon screw inwards towards the inlet valve. then tighten the locking nut to stop it moving,
Good luck...
Peccavi
March 2010
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