When you say that it is over flowing to the drain, is that the main valve or the brine tank?
If it is the main valve then either the main piston and seal assembly has a seal out of place , the piston has a grove are the only two real items that will have the unit dumping water to the drain when the unit is in service.
If it is over flowing the brine tank then there is a plug in the injector assembly area, most likely the injector and screen are pluged.
Culligan is looking for 150.00 at least for the rebuild kit that has the main piston with seal pack and the brine piston with its cage.