Parker's Hydraulic Quick Couplings have a wide variety of designs, some of which are tailored to a particular application or use.  Based on the valving, hydraulic couplings can be either double shut-off or straight-through.  Double shut-off couplings have valves in both halves (coupler and nipple) keeping fluid loss to a minimum.  Straight-through couplings have no valves and are ideal for high flow with low pressure drop.