The release valve must is open. Is the valve stem, knob damaged?
Can you rotate / turn the piston rod? When trying to rotate the rod, is there on area where it is harder to turn? The piston rod may be bent if does not rotate freely.
Are you pulling the rod with enough force? When the Integral is new, everything is very tight and requires a lot of force. For example the smallest Integral, -08 needs 30 lbs. of pull to break away friction. A -30 needs over 200 lbs.
To manually gain purchase, remove clevis pins and thread a rod (a huge screw drive or metal dowel) through the clevis pin hole. Do this at the top end (Upper Jaw) and at the bottom end (Lower Jaw). You can plant your feet on the bottom dowel. This makes it easier to pull the rod out.
If the piston rod is "frozen" that is probably due to galvanic corrosion where the stainless rod "adheres to the aluminum cylinder. This happens when the wiper seal which sits on top of the cylinder and clean contaminates off the rod when it goes into the cylinder is
worn, cracked, extruded or simply missing.
This Integral needs service now.