tap pressure is NOT the same if you don't open the tap alla way, a flow restrictor is a pressure restrictor, put a pressure gauge on it if you don't believe that ... or just try it on your lawn sprinkler
as Stuart sez, if it has the thermostatic capillary tube on it, you might want to check that, but it's the capillary tube that needs to be heated Stoo, and if it's not broken and the guts of the valve aren't corroded, it'll work .... the leaking part is the main problem those things develop, thermostatic or manual, and water is the test, not air - I've rebuilt 2 of them cuz of leaks with the kit I cited above
Bill