The rear brakes in my hobby car are drums type. One of its brake cylinders have one of the pistons "really" stuck: it won't move by hydraulic force by pressing the brakes pedal; it won't move by slightly tapping it in or out of the cylinder body and didn't even moved after I caller the services of my Margarita (10 lbs hammer). It is really stuck...so what other thing I can try? Heat it? Freeze it? The body is cast iron, the piston is aluminum.