How to Fix Strong Suction on a Flapper
The toilet flapper is the rubber mechanism in the middle of the inner tank bottom. It allows water to drop down from the tank to the bowl when you flush the toilet, and it then shuts the water off. An amount of suction allows the flapper to seat properly and prevent further tank water from sneaking out. Other parts cause too much suction on the flapper, preventing it from raising fully enough to let enough water flush out the bowl contents.
Remove the lid from the toilet tank, and set it safely out of your way, so that you won't trip over it or possibly damage it. Locate the rubber flapper, which is usually red, inside the tank.
Examine the lift chain that runs from the top of the flapper to the metal or plastic trip lever. The chain length should allow only a slight amount of slack.
Remove the chain's metal clip from the trip lever. Attach the metal clip to a lower link on the chain, and reattach the clip to the trip lever; or, cut off excessive links from the chain with a pair of snips, and reattach the metal clip and chain to the trip lever.
Flush the toilet, and observe that the flapper raises properly, without too much suction preventing it from doing so. Place the lid back on the tank.