I have three toilets in my house. Each of them sometimes flushes more than once, but the upstairs toilet always flushes three or four times. The house (and plumbing) is about 18 years old. I replaced the original float mechanisms with the cylinder on a shaft mechanism a couple of years ago and that seemed to fix the problem until it reoccurred about six months ago. I know that the water pressure in my house is high (I really like that for showers), but could that be a contributor?
Ronald A. Wilson