I changed my front disc brake pads today. The old pads on the left side were less worn out than the right side pads. I also had hard time pushing back the left piston into the caliper with a C-clamp. Right side piston was easier to push back with the C-clamp. After replacing the pads I still have to press the brake pedal hard to stop the car as if the pads were old. What is causing this? Do I have air in the left side brake line or is it a bad piston?