Hydraulic cylinders carry a bulk of the workload of many hydraulic systems. They often require maintenance. Repairing a hydraulic cylinder usually involves replacing all of its seal. You can't repair hydraulic cylinder seals; you should reseal the entire cylinder whenever you replace one seal. Since you have to disassemble the cylinder to access any of the seals, repairing one seal means uninstalling the cylinder from its attaching parts, completely disassembling it and replacing all its seals.
Open the cylinder ports and empty all of the cylinder's hydraulic fluid through the ports.
Secure the cylinder on a bench vice and fully extend the piston rod.
Remove the retaining nut, head nut, snap-ring or other device that holds the rod end steady and in place. Examine the cylinder and make sure you have the right tools to remove the retaining device at the rod end. You may need snap-ring pliers, a spanner wrench, a flathead screwdriver or other tools.
Pull the rod completely out the cylinder barrel to expose the piston and any other attached components.
Bring the rod end assembly to a soft-jawed vice and secure it. Unscrew the nut holding the piston in place with the appropriate sized wrench or socket, and then remove the piston and any other parts attached to the rod.
Lay the piston rods and all parts on a clean work area. Remove all the seals with a seal removal tool.
Clean the inside of the cylinder barrel, the piston rod, piston and other attaching parts with a petroleum-based solvent, then air dry them.
Inspect the inside of the cylinder barrel, piston, rod and other polished parts for burrs and scratches. Smooth areas as needed with an emery cloth.
Install new seals on every applicable part. The cylinder seal kit will contain every seal to be replaced. Install them in the same manner as the seals that you removed. The piston seal on some cylinders may require special installation steps. Consult the cylinder maintenance documentation if you think this is the case.
Place the rod back into the soft-jawed vice, and install all the parts back onto the cylinder rod in the reverse order of how they were removed. Install the piston last, then tighten the piston nut to its specified torque with a calibrated torque wrench.
Coat the assembled piston rod and resealed parts with hydraulic fluid, using a clean shop rag or a gloved hand. Insert the piston rod back into the cylinder barrel.
Reinstall the retaining device to the rod end of the cylinder. Screw a gland nut or spanner nut securely onto the cylinder using the appropriate speciality wrench. If the cylinder is secured with a snap ring, compress the ring with snap ring pliers and insert it into the groove at the rod end of the cylinder. Release pressure from the pliers to set the snap ring into place.