I know the right way to do. Replace the seal. That will mean.
1. cut rusty screws off wall.
2. repair crumbly concrete wall.
3. tank falls apart because of hairline crack.
4. Pipe leading from tank to bowl breaks.
5. Part of toilet where pipe connects breaks.
6. Replacing bowl reveals flange is corroded beyond use.
7. Wood around flange needs to be replaced.
8. lead flange breaks off in concrete floor.
9. Remove floor, joists to repair floor.
10. Put it all back with new toilet.

The seal between the valve seat and the tank got toasted when I tried to sweat the fill pipe in after it snapped off.
I know it was stupid.
Now it leaks and the seal is brittle and crumbly.
I filled it and let it sit a week thinking it might swell and stop leaking. It did not.
I drained the tank and it has been dry over a month now.
Please don't tell me to replace the toilet.

I was thinking that a PVC pipe rubber cement would be viscous or thin enough to seep into the seal and the gaps in the seal. Two or three applications of this with a baby medicine syringe to get it where I want it.
I would then silicone it over that.

I have some options.
1. Silicone. Too thick to saturate the dry seal.
2. PVC pipe cement. Thin enough to saturate seal.
3. Contact cement. Not the right stuff.
4. Epoxy. Too brittle tank moves a little.

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