SUN three port counterbalance cartridges (with pilot-to-open assist) are modulating devices that allow free flow from port 2 (inlet) to port 1 (load) and then block reverse flow until a pilot pressure inversely proportional to the load pressure is sensed at port 3 (pilot). They typically employ a check valve for free flow in one direction, a relief valve to control flow in the other direction, and a pilot that lowers the relief valve setting with pressure. Thermal relief protection is provided in most systems. Deceleration control can be provided when used with open center directional control valves.