Toilets running water

[Kar Sun]

I have 3 toilets in my property one above the other on 3 floors. Suddenly 2 started running water and 3 rd one started making odd sounds when flushed. The toilets are Eljer and are about 15 years old. Only one tenant on the property. The entire mechanism inside water tanks has been replaced with the standard valve and flapper. The issue appears to be fixed after the install and test. Then tenant reports in a few days that the issue is back. I really do want to replace the entire unit but what might be the case? Thanks