I am trying to chang my back brakes. I got them off the part that I am having a problem with the piston. I have the blead valve open and some oil comes out of it. I can't push it in all the way with my hands. Should I be able to push it in by hand or should i use a C-clamp or is there something else I should be using?.

 

take a large adjustable pair of pliers and clamp the caliper and the piston between the claws. gently squeeze until the piston is back far enough within the caliper to provide clearance for your new pads to fit over the rotor. You shouldn't have to open the bleed valve to accomplish this.