Leaking hydraulic cylinders aren't hard to fix, as long as they aren't: (a) 6-inch diameter by 36-inch long wing-fold cylinders off a mammoth field cultivator or disk, or (b) Wire-Lok-type cylinders that use sometimes tricky internal lock rings to attach the end cap to the cylinder bores.
If the leaky hydraulic cylinder is a simple 3-inch by 12-inch cylinder with the end caps held against the bore by four long "tie-bolts"---it's a piece of cake. With some basic tools and a large dose of patience repairs are straight-forward:
First, identify the make and model of the cylinder and acquire the proper seal kit. Many cylinders have model numbers stamped somewhere on them. Otherwise take the cylinder to your equipment dealer and let the expert at the parts counter decipher what you need.
Second, power wash or use a parts washer to thoroughly clean the exterior of the cylinder--top, bottom, sides and ends. Clean an area of your workbench and cover it with an old feed sack or a layer of paper towels. "Clean" is the byword when working on hydraulic components.
Clamp the cylinder in a bench vise. Use a cold chisel or center punch to make marks on the end cap and cylinder bore so you can reassemble those components in proper alignment. Use an air impact wrench or hand tools to remove the 4 nuts on the tie-bolts that hold the end cap through which the cylinder rods slides in and out. Put a drain pan under the cylinder. Use a hammer to tap the end cap out of the cylinder bore. Once the end cap pops free, pull or tap the clevis on the end of the cylinder shaft to remove the shaft/piston from the cylinder bore.
Carefully lay the shaft/piston assembly on the (clean) bench top and take a deep breath. Note the arrangement of seals, o-rings, backing rings and other sealing components on the end cap and the piston. If necessary, make a sketch on scrap paper of which o-rings and back-up rings go in front of each other. Open the seal repair kit purchased at the dealership and carefully lay out all the o-rings, back-up rings, etc.
Use an impact wrench or hand tools to remove the lock nut that holds the piston to the shaft. Note that there is an o-ring inside the piston. Slide the shaft through the end cap. Note that there are seals/o-rings inside the hole in the end cap. Pay close attention to the orientation/sequence of those o-rings and seals.
Now carefully use a small screwdriver or o-ring pick to systematically remove o-rings and back-up rings and replace them with new ones from the kit. It helps to lay out the old parts in-order, exactly as removed, so you can refer to them about positioning and orientation. Be aware that some "flat" back-up rings actually have one surface that is subtly curved to nestle against its o-ring, and that some back-up rings have chamfered (beveled) edges, so they must be oriented correctly.
Seals in end caps--the ones through which the cylinder rods slides in and out--can be challenges to remove. Use care not to damage the often soft metal of the end cap. A very small cold chisel or sharp metal punch can often help pry out those stubborn metal-framed seals. Before you remove (destroy) the old seals be certain to note the "front" and "back" (inside or outside) of the old seal so you can install the new one correctly.
With all the new seals installed, coat every component with light grease or hydraulic oil and carefully, carefully, slide things back together. As you push the piston into the bore, and as you tap the end cap back into place, use a small screwdriver to gently compress o-rings and seals into their grooves. It often helps to stand the cylinder on end so the weight of the piston and end cap works to slide those components into place--all you have to do is use the mini-screwdriver to compress any protruding seals or o-rings. Even the best mechanic occasionally pinches an o-ring or seal during re-assembly, so don't take it personally if that happens.
Once the piston and end cap are installed, tighten the nuts on the tie-bolts to approximately 200 ft. lbs. Repairing leaky hydraulic cylinders is an oily, sloppy job, and rebuilding mega-cylinders if often a two-man job simply because of the weight and size of the components. But it's not brain surgery, so most farmers can accomplish the repairs themselves if they have patience, a few tools and don't mind getting well-oiled in the process.