Mansfield 35 flush valve or seal leak

I recently replaced the flush seal (the red one) and the fill valve for a Mansfield 35 toilet that is 18 years old. After the tank filled I shut the water supply off. Even though I couldn't hear it refilling or leaking, the tank is empty when I check it the following morning. Should I install a new Mansfield tower style flush valve or can I replace it with a Fluidmaster 507A or 507C flush valve? There doesn't seem to be a good seal with the flush seal. Thanks for your help.

mschloss said:

...the tank is empty when I check it the following morning.

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Maybe you sleepwalk? The textbook test is to put some dye in the tank.

since I have not had that problem with a Mansfield toilet, I have to assume you did NOT position the seal at the bottom of the tower properly, or your seal is warped or otherwise defective.

I made sure the seal was on correctly - placed on the upper ring. I thought that maybe the overflow tube was siphoning the tank so I pulled the tube out of the overflow last night. The tank was empty in the morning. I guess I'll buy another seal and try it again. If that doesn't work I'll replace the flush valve. If I do replace the flush valve would you recommend replacing with the same Mansfield tower flush valve or installing a different brand with a flapper instead? Thanks for the help.