Abstract: Disclosed is a packer (32) responsive to a first hydraulic pressure for setting the packer (32) within a casing (16), and responsive to a second higher pressure for releasing the packer (32) from the casing (16). The first pressure moves an inner piston (94) which shears a screw (96) to thereby apply a compressive force to elastomeric bands (66, 68). The elastomeric bands (66, 68) bow outwardly and seal the packer (32) to the casing (16). A toothed ratchet ring (135) engages with respective teeth (114,122) of the inner piston (94) and an outer cylinder piston (90) to permit one-way movement of the inner piston (94). The packer (32) is thereby set and the hydraulic pressure can be reduced. The packer (32) is released from its sealing engagement with the casing (16) by applying a second greater hydraulic pressure which shears a screw (92), thereby allowing the outer cylindrical piston (90) to move downwardly.