When I flip the handle to flush the toilet, the flapper drops back to the hole before only a small aount of water has entered the bowl. In order to get a complete flush, the handle has to be held down for about 10 seconds. I've changed the flapper, but it did not make a difference. My plumber tells me that the toilet is old and needs to be replaced. This is a low-boy water saver about 20 years old. Any ideas would be appreciated.