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.