Try removing the conical shaped metal spring from above the piston assembly - don't worry this won't cause it to leak - also check that the centre column isn't being pressed down all the time while the lid is on. Failing that you may need to replace the washers; these can be got from sales@insumo-uk.com
plumb answers
November 2008
Automatic Answer
While you wait for answers, we may have found some information which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix toilets.