I recently overhauled our TMC unit but it is not timed and relies on the old electric
manual push button. Exactly the same problem which was also occurring in the old unit as well. We found a simple fix. The water
being pumped in was greater than the water
being pumped out, we just adjusted the gate valve reducing the flow into the toilet till the water was at the same level as the other TMC units in the boat. We started at shutting the valve halfway and adjusting it accordingly. Our valve is not set at roughly 3/4 open and it has worked well for the last 9 months no issues. Hope this is of some assistance.
Greg and Sue