What could be the cause of a one piece Kohler toilet, 30 year old model Rialto 3402, a 1 piece toilet tank to completely drain when the source water valve is turned off. When the source water is turned on and the tank if full there is no disturbance of the water in the bowl but looking into the round stand pipe you can see the water running. How can I fix this problem

Thanks for your help.

Sounds like the flapper is bad or possibly the seat. Fill the tank and put a couple of drops of food coloring in the tank. Look in the bowl. See color? Bad flapper. Note that some Kohler toilets need a Kohler flapper and will not work with a universal replacement. Take the old one with you.

ma0641 said it: replace flopper.

Be careful, low-boys Rialto toilets take specific flopper. Go to plumbing supply house that caters to professionals. They'll have it in stock - otherwise, in big box stores you get only something similar to it and it will not fit properly and will leak again.

Hope that helps. Back to you. Milo