Culligan control valve question

Have a 15 year old Mark 89. Suddenly in service position, has developed constant flow (barely salty to my taste) to drain, at 22 oz/min. Ceases only when unit bypassed. Valve has a hissing sound (air leak?) when draining, but couldn't detect any external leak. Watched the cam, plungers during recharge cycle, and each phase seems normal length and function; back wash and resin recharge seem normal,as does rapid rinse.
Service here has been good at allowing me to self-repair other problems, often with free parts. They're real busy these days and not really interested in troubleshooting - just set up service call for two weeks away!
Any ideas? Thanks in advance.
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