I have a Kenmore Deluxe Demand water softener (model 625.348591, 5
years old) that is leaking water into the brine tank when in service
mode. The regeneration cycle appears to be working fine. But in
service mode it looks like water is leaking from the main valve
assembly into the tank. After a few days, the tank fills up and water
leaks on the garage floor through the overflow drain.

The nozzle and venturi look okay. No clogging or dirt.

Is this a simple valve gasket problem or does it look like a more
serious problem? Anybody have a similar problem like this?