Fuel Selector Valve

We were doing the annual on a 1973 Cessna 172M today and discovered the fuel
selector valve would not shut off the fuel - in any postion. The handle is
not slipping on the shaft and there is mechanical connect through the
linkage. The detents can be felt in each position and we can visually see
the component turning on the exterior topside of the valve assembly. So the
valve has to come out so we can examine it thoroughly.
Anybody experienced this?
Tom Cummings