Replacing a dual flush valve seal on an American Standard toilet is easy. Follow these steps:
• Turn off the water supply to the toilet.
• Remove the top cap of the tank, and unscrew the retaining nut around the flush valve.
• Pull out both gaskets from inside and replace them with new ones.
• Put everything back together in reverse order, making sure that all connections are tight.
Finally, turn on the water supply again and test if it works properly.