I have 3 mansfield 160 toilets; 2 of them were becoming hard to flush so I changed out the flush valves and then the gaskets but they are both still leaking and I can't figure out why. I took the parts off and compared them with the one mansfield toilet with the original parts to make sure they were correct and they are. I made sure the gasket was sitting where it is supposed to be and it appears to be correct. The only way I can get them to stop leaking is to push down on the flush valve with my had but it only stays that way for a few seconds and you can visibly see the flush valve pop back up. I'm not sure what to check next. I am having to shut the water off between each use. Any advise. The toilets are only 3 years old.