Kenmore Water Softener leaking water in brine tank

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?
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Very similar to my problem. How did you determine if it's from the main valve and not the resin tank? I have read that the seal kit for the main valve is about $40, but the resin tank is $400, so it's kind of important. Ours is a little over 10 years old, model 625.348732.
Ed

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Initially it looked like the leak was only at the outlet side of the main valve where it connected to the bypass valve (a single plastic bypass valve manifold). It was dripping out directly below the plastic clips that hold the valve to the bypass valve, which made it look like the o-rings were mal-functioning.
After replacing the o-rings on both the inlet and outlet side and slowly bringing the system back up to pressure, now, both the inlet and outlet sides were leaking. (This is pretty typical for my repair projects. Things usually get worse. Then I'm pressured to pay someone to do what I know I could have done myself.) I used the correct size o-rings, and I inspected the valve prior to reconnecting for cracks. But I'm not sure I would have seen them without pressure on the system.
Anyway, now I have dripping inlet and outlet ports on the main valve and I'm not sure what to do now.
Gary

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