The other day when steaming milk it occurred to me the valve (handwheel) was not as free/ loose as usual and similar on the water valve. N.B I never screw them off tightly until they touch bottom. A job when M/ch is cold.
I removed both valve inners after prying loose the blanking caps PIA.
Valve stems and seals were fine.
The threaded parts which actuate the stems & seals were coated with a hard wax like material.
I can only presume this is the residue left after the lubricant has been driven off by the high temperatures of steam and water.
A thorough cleaning with meths and reapplication of Mol 111, reassemble and movement as smooth as silk again.