A male and female set of hydraulic coupler cartridges mate such that one part of the coupler can be left in place, e.g. the female coupler, with the other portion of the coupler, e.g. the male coupler, being removably connected or disconnected. Valves within each cartridge comprise an actuator and springs within each cartridge urge the valves into a closed, sealing position. When the cartridges mate, spring and/or fluid pressure within the cartridges operates to either allow or disallow fluid flow. Once the cartridges are mated, their respective springs are partially compressed and movement of one of the valves operates to compress one spring and correspondingly decompress the other. The cartridges can therefore be mated and unmated, e.g. subsea, while fluid flow is prevented through them during the mating and unmating.