DIYbanter - Simmons Temptrol Shower Valve Leakage

Hi,

Have the pretty typical Simmons Temptrol (single large knob) hot/cold water
adjusting control in my shower.

Have a leak in the plaster in the general vicinity of underneath the shower,
but not exatly.

Realize this is a tough one to answer, but what is the "history" of, and
experience with, these units actually leaking
to the outside ?

By this I don't mean allowing leakage thru the shower head, but actually
leaking to perhaps
the plaster in the room underneath.

Was wondering if there is a history of this, or they are pretty reliable in
this regard ?

Hate to start digging it out if it isn't the actual culprit, or likely to be
the problem.

Thanks,
Bob