Toilets, Sinks, Showers, Tubs and Disposals - Toilet leak single piece toilet

I have a old single piece toilet that keeps slowly leaking water into the bowl. I used food coloring to check. It's an old toilet with the overflow tube molded into the corner of the tank. The flapper assembly is a wierd looking thing. I have replaced everything except the brass seat for the flapper (can't find one at hardware store and don't know how). Can't hook refill tube to anything because overflow tube is molded ceramic. I don't think it is siphoning into bowl.

sulmar, Welcome to the DIY Forums.
First off, try to find a name on the toilet This may help you get parts. Turn off the water supply to the toilet and flush it. Lift up the flapper and look down inside it. There should be a nut (probably 1/2" or 7/16"). You will need a socket and probaly an extention. Being careful not to bang the ratchet on the bowl, remove the nut. There should be a piece that goes across the hole that is holding the flapper mechanism in place. Don't lose the nut. Sometimes you can just loosen the nut and wiggle the piece around and get it out of the hole. Once out, check the seating surface and buy a new rubber seal that sits under the brass. Reinstall. Good luck.

No nut, Three tabs with screws. I guess loosen screws and pull out. So this thing is probably leaking from the gasket and not the third flapper I put on. Makes sense. Thanks.

I replaced 4 flappers and even bought one that had a soft seal that replaced the brass seal of the original. No luck. Then I found the crack in the overflow tube right where it connected to the drainhole on the flapper seat.

The overflow tube is molded into the corner of the tank away from the flush valve mechanism. No detectable cracks. leak appears to be coming from flush valve.

Must be the rubber seal under the flapper unit then. I can think of no other reason for the leak.

Replaced whole flush valve assembly. Now all parts are new. So far, so good. Thanks for all your advice.