All toilets since 1990 are required by law to use a maximum of 1.6 gallons of water per flush. It is normal for some water to remain in the tank after a flush. The reason for the extra water is to help add a good amount of pressure for the flush. Flush valves are engineered to only allow a predetermined amount of water out before they close. A flush valve can go bad, and close too soon. This can be caused by a damaged flapper, flush valve seal, or a cracked flush valve.
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