A hydraulic pumping system can include an actuator including a piston rod that displaces in response to pressure in the actuator, a seal assembly that seals about the piston rod and is exposed to the pressure in the actuator, and another seal assembly that seals about the piston rod, is exposed to pressure in a well and includes multiple separate seal cartridges, each of the seal cartridges including a dynamic seal that slidingly and sealingly engages the piston rod. Another hydraulic pumping system can include an actuator including a piston rod that displaces in response to pressure in the actuator, a seal assembly that seals about the piston rod and is exposed to the pressure in the actuator, and another seal assembly that seals about the piston rod, is exposed to pressure in a well and includes a labyrinth ring comprising multiple ring layers.