I know this is an old thread but in case anyone reaches it through the search function the answer is probably a bad valve pack. I just replaced the valve pack (not just replaced the O-rings). It was $100 from Culligan + $6 for 4 more replacement o-rings for the fittings between the head and the bypass valve fittings. Disassembling the head took about 15 minutes. Take pictures of what you disassemble so you know how to put it back together or watch Youtube videos. I didn't find any on how to take the head off the resin tank and forgot how I did it 9 years ago when I last had the valve pack replaced. Then I remembered. You take the screws off the metal bracket holding the head to the rear pipes, then the head slides off the top of the fittings of the tank and off the fittings in the rear. It's just a fraction fit between the head and the fittings on the top of the tank.
Remove the brine hose (threaded nut) and outlet hose (wide horseshoe clip)
Remove the long narrow horseshoe clip in the rear of the head and use a section of a 1" PVC pipe to knock the valve pack out of the head. Hold the head unit between your legs, put the pipe into the hole and use a mallet to drive the valve pack out either from front to back or back to front. It's a smooth friction fitting but very snug so be careful not to damage the walls of the head. There is a disk with an o-ring to stop the valve pack from shooting out the rear being held by the horseshoe clip. Slide it back to the rear and replace the horseshoe clip.
The black valve pack, white needle valve housing and the needle valve are all pre-lubricated so you don't need any additional silicone grease. The o-rings for the rear fittings are not so I bought a $3.50 tube of plumbing silicone grease to lube them before installing.
Putting everything back together took another 15 minutes. Just be sure to align the needle valve on top to the cam housing and the bottom plastic plate to the pushrod and bottom camshaft button. The motor is strong enough to strip and break the plastic pieces so it's important that they slide freely.
Also take the 3 screws off the top metal plate, remove the plate and make sure the eductor (brine sucking) parts are clean especially the little hole at the tip of the plastic port.
A Culligan service call will add about $100 to the replacement cost of the valve pack but if you aren't mechanically inclined, it's well worth it. The first time I had Culligan come in to do the work, the second time, I took the entire head in to the Culligan store and had them rebuild it. This time I did it all myself. 3 times in 30 years isn't too bad and I'm still getting good service out of this water softener. The only other service I do to it is to rise out the brine tank every couple of years.
