Flush Valve Seal wont Seal

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There are lots of reasons a flapper will not seal properly.

The flush valve seat that it rests on might be pitted or dirty.
Very common problem.

The flush valve is leaking at the bottom of the tank, around its gasket, and the flapper is not actually the problem.

You have the wrong shape or style of flapper for that flush valve.

The flapper got deformed while sitting around on the shelf, so it is not laying like it should be, perfectly in contact against the flush valve seat.

The flush lever chain is too tight, the clip at the top of the chain is getting hung up, or you are hanging the chain from the wrong mounting hole on the flush lever, so it is getting pulled off-center.
 
How old of a Mansfield toilet is it? Does it take a flat red seal? Or does it take the flat black seal?
 
However, the new valve seals seem to be stiff and do not make a tight seal with the plastic "cup" ... If it is deformed, you will never get a good seal and will have to replace it.
 
If he gets a genuine Mansfield seal he should be ok. The knock offs is where you have a problem. I just put one on for a
customer yesterday and it fit perfectly. Like CT18 said , if you don't get it on the grove correctly it is going to leak
for sure.
 

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