A hydraulic actuator which may be used as a surge brake actuator may include a housing defining a low pressure chamber and a high pressure chamber, a low pressure piston assembly which is in the low pressure chamber and has a contact surface, a high pressure piston assembly which is in the high pressure chamber and which moves forward in response to forward movement of the low pressure piston assembly, and an unloader valve in fluid communication with the low pressure chamber and having a closed position and an open position adapted to allow hydraulic fluid to flow out of the low pressure chamber through the unloader valve, wherein the contact surface engages and pushes the unloader valve from the closed position to the open position during forward movement of the low pressure piston assembly. The actuator may be used for actuating a brake system.
