Remove the Brine Valve assembly (float assembly) completely from the brine well and hang it outside of the water softener (assuming it is the cabinet style 3000 series) and set it in a pail or container. Check to see if water constantly flows from the brine valve asm. while in the service cycle. If water leaks constantly from the assembly, then you have to rebuild the control valve (replace rubber seals and possibly the rotor & discs). If no water flows from the brine valve asm. Check to see if you have a slow leak from the resin tank. With the brine valve assembly still in the pail, bail some water from the brine tank to about an inch below the overflow level. If water rises to the overflow level again (may take a few minutes) , then you have a slow leak from the resin tank. If there are no obvious signs of an external leak (water dripping down from the top of the resin tank, then the leak is at the bottom and the tank will have to be replaced.
Hope this helps...