Flapper closes to soon - American Standard Colony 2388.012.222 Linen Two-Piece Round Front Toilet

replace the faulty seat seal in the cistern

Try adjusting the water level in the tank. Most flappers close as the water in the tank flows out to a certain level. Try allowing more water to come in to the tank. I doubt the weight of the chain has any effect on it closing too early.

Was the flapper chain adjusted properly? If too tight it will not sit flat to close the hole. If too loose will not flush properly. Also check the rim where the flapper valve closes for any cracks, nicks, flat or broken spots. Any of those will keep the flapper from sealing.

Fill the breather hole (1/8" hole) inside the flapper.This is designed to let air out so the flapper will sink. Now it floats longer letting in more water. 25%percent more in my case.

Just increase the length of the chain so the flapper valve does not have time to close too soon.

If this is a standard toilet, take the lid off the tank (the back part) and adjust the length of chain that runs from the flush handle rod down to the flush flapper. Usually if the chain is too long it will stop flushin too soon. It also could be that the rubber flapper valve in the bottom of the tank is worn out and needs replaced. (any hardware store).

lift off cover and shorten flapper to handle chain/rubber connection flapper not opening all the way

Which style of flush valve do you have? If it is the dual-flush option (solids/liquids) I do not know how to adjust those. If you have a traditional style, then the flapper or ball is closing too soon. You need to adjust the guide arm (flush ball) higher or shorten the chain between the flapper and the handle. Adjust this carefully or you will have it fail to seat and the toilet will constantly run.

I hope this helps.

Cindy Wells
(who needed to replace the fill valve and the flapper at her home earlier this week)

These models need to have a flush valve flapper that has a styrofoam float on the lift chaninof the flapper. Go to hardware store or home improvement store and get a 'Korky brand toilet tank flapper with float 'ball". Cut the rubber ring piece away from the flapper and you don't need it (see cut here marks on flapper). Install flapper and adjust the float fairly close to the flapper.
When the water level drops in the tank and reaches the level of the float on the opne flapper, the flapper quickly closes causing the toilet to have a "surge" of siphon action which forces the water in the bowl to be drawn out.