32 reduces the pressure in the casing annulus 30 and also reduces the pressure differential between the easing annulus 30 and the tubing annulus 32 removing the pressure different allows the spring 77 to reseat the valve seat 72 and reinstate the sealing interface as illustrated in fig2 this would be typified by a nominal relief setting of 500 psi on the valve the actual value being predetermined by operator preference in one example of use when pressure in the casing annulus 30 approaches a designated pressure that may potentially damage wellbore assembly 10 hardware the vent valve 52 automatically reverts to the open position of fig3 casing annulus 30 vented into tubing annulus 32 until pressure in the casing annulus 30 is below a potentially damaging pressure the casing annulus 30 is vented until the pressure therein is no greater than 500 pounds per square inch or some other value of the pressure setting of the valve 52 less than the minimum differential rating of the wellhead assembly 10 and surface casing 18 when considered together optionally the pressure could be reduced yet further for instance down to ambient pressure in an attempt to compensate for a slow leak downhole past for instance a production packer not shown or tubing joint as a contingency later in field life if desired during for instance.