Step 3 · Inspect the Lift Chain and Flapper
Inspect the lift chain that connects the flush handle arm to the flapper. The chain should have a slight amount of slack, about 1/4 to 1/2 inch. If the chain is too long (too much slack), it won't lift the flapper high enough to release the full volume of water. If it's too short (too tight), the flapper won't be able to create a proper seal on the flush valve seat, causing a slow leak from the tank to the bowl. Adjust the chain's position on the flush handle arm as needed. Also, check the rubber or silicone flapper itself for signs of warping, bubbling, or deterioration, which can also prevent a proper seal.