An improved mechanical system selectively locks the tester valve of an annulus pressure responsive tester valve in position for an indeterminate number of well annulus pressure cycles. The tester valve can be closed upon demand. The forces which accomplish opening of the ball valve act across a power piston, but the forces which close the valve act across an actuating piston. The tester valve can be run into a well with an operating element of the tester valve, in a first position such as a closed position. Upon reaching the desired depth within the well and setting of an associated packer system, well annul us pressure is then increased to a first level above hydrostatic pressure to move the power piston and thus move the tester valve to an open position. During a normal mode of operation, well annulus pressure can be cycled between hydrostatic pressure and the first level to open and close the tester valve. A fluid transfer assembly is included within the power piston which is operable to transfer fluid across the power piston. The fluid transfer assembly also includes an oppositely disposed check valve to allow unrestricted fluid flow across the power piston in the opposing direction during a release of annulus pressure. The tester valve also features a multi-range metering cartridge which is operable to meter fluid over a wide range of differential pressures.