#1 you have to make sure the parking break is off.. although to be honest i don't know how you got this far if it's not.... otherwise it will keep pressure on the piston. (make sure she's in gear and not going to roll anywhere-like off the jack)
#2 As it's been stated, the rear piston should screw down.. i've done it with a flat head screwdriver before... most have a x or plus sign indention on the piston that the whole bar of the flat head can fit flush sideways.. and then give it a crank (i think counter clockwise)
#3 i've done an acura that didn't screw and i didn't have a clamp... turns out a three cell maglight handle fits perfectly in the sleeve and i pushed with my knee... good luck...
#4 worse things could happen...freakin stripped guide screws ..etc.