I recently installed this in my new bathtub. the valve has a single handle with a knob on the tub water spout to redirect water to the shower. Here is the problem. When I turn on the cold water at first all is good. As I continue to turn the handle to the hot the pressure increases and the water starts to drip from the shower head. If I turn the water more toward the hot usually this will usually stop but not always. Is this correctable or am I going to have to replace whole thing? Any ideas would be appreciated thanks.