Mansfield Aegan 705 Toilet Leak

I have two Mansfield Aegan 705 toilets which have the "newer" style flush valve. In other words they do not have a floating ball. I have tried using parts from the other to determine the source of this problem and I cannot quite figure out where to go next. The toilet is leaking around the seal where the top of the flush valve drops down to make a seal. If I press on this piece and apply pressure, I can stop the leak. I have used the part from a working toilet and the issue persists. I have also replaced the rubber seal that is placed around the ring. Is this an issue with the seal, or is there a pressure issue in that the valve is not being held down firmly enough. I have checked the length of the chain that raises this component as well and it is adequate in length. I have had two plumbers look at it and they both do not know what to do next, aside from replacing the entire toilet. I assume there is some kind of sealant I can use because there is a clear sealant type substance around all of the tank as it is. I just do not know what this substance is. Any advice would be perfect.

Thanks.