One thing I have learned is sometimes junk can get in those fluid master valves when the water is turned back on, It always helps to pop the top off and makes sure all is clear. I generally just replace toilets that are old. I get Gerber viper toilets for 100 dollars .
One mistake made a lot of times with replacing the seal between the water tank and the bowl is that some people tighten the bolts too tight which can actually distort the gasket. YEP ! The bolts between the tank and bowl should have the rubber washers under the bolt heads, the metal washer should go up under the bowl. The hole for the bolt should be a bit larger than the bolt and the washer will allow the nut to have more seating surface, I like to add a couple of extra metal washers so that they do not bend. Turn out the light in the bath/toilet area, use a flashlight to view between the tank and bowl while flushing the toilet. Look at both sides carefully. IF this is where the leak is (and I'm betting it is) you will see the water run out this locating the problem.
I had same problem. Fixed it! Water was leaking between bowl and tank only when flushing. I had changed out the big gasket about a year ago so it was still fairly good and should not have been leaking. It had a 2 inch tank to bowl universal gasket and I could not get it to seal, so I kept the 2 inch gasket on and put an additional 3 inch gasket on as well. It seals like a dream now and I didn't have to crank down on the bolts to the point of the tank breaking. Might be overkill but it works. Hope this helps someone.
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