My bathroom shower is leaking. I checked the shower head. Shower head is fine...water is leaking from the pipe itself. When I turn the Hotwater/Cold water knob completely towards the cold side, dripping stops. But, if I keep the knob in the center or turn towards the hot side, water starts dripping. My house is 12 years old, and the knob fixture is made by Price fister. It is single head, round type. My problem is, I don't know how to open the knob since there is no visible screw in the center (flat metalic cover). Any help to fix this problem is highly appreciated.