[Motor31]

First thing to do is to clean the flapper valve. That means doing a nasty job. You can try a soft toilet brush. Open the valve then start scrubbing around the seal area. There was a small tool for cleaning the seal provided with the toilet. It was a small flat white plastic strip with a soft scrubbing surface on one end. You leave the valve closed but insert the "scrubber" between the seal and valve and scrub all around.

If neither of those fixes it you may have a tensioning device behind the front screen under the bowl. Take the front cover off of the base and look for a screw band and increase the tension a bit. If that doesn't work you're stuck replacing the flapper assembly or the toilet.

I haven't had to do anything but cleaning yet and have fingers crossed.