not a problem, hopefully the leak will be a simple repair even if its not really the valve itself what I would do is try to trace down the leak first, it is very well possible the mechanic did not tighten down the hoses quite enough or possibly may have not changed one that may be cracked and leaking, possibly a needlepoint sized hole in a radiator fin. several possibilities if the leak is coming from the front i would rule out the monovalve.
Though winter is coming so It may be a good idea to change it now (if it's going south)
one good way to tell if its bad is go for a drive one day when its cool out (late at night early morning) turn on the heat. if you turn the heat off while its running then turn it back on but no heat comes out then the valve is more than likely bad also another way is once driving and the RPM's hit 4000 the heat should cut off altogether causing you to have to turn off the engine to restore heat....