Repairing a leaky hydraulic hose on a planter is easy if (a) the engineers designed the planter with all hydraulic hoses running outside of frame tubes, or (b) the planter is a mono-frame design--as in, it doesn't have wings that fold up or forward.
But if a planter's hydraulic hoses are routed through the frame tubes, and the planter is a folding planter, it can be a challenge to repair a leaky hose. First, because it may be difficult to peer into the oily mud clogging the frame tube and determine which hose is the culprit, and second, because pulling the hose around, through or over a hinge assembly can be a major pain.
Patience and a pressure washer can help determine which hose is leaking. Pressure-wash all the accumulated goo from inside the frame tube, let it sit overnight to dry, then operate all the hydraulic functions, one-by-one, and check to see which hose is the source of the leak. A small, powerful flashlight is a big help.
Once the culprit is identified, figure out where to unhook both ends of that hose, diagram its location and head to the dealership to get a new hose. My preference--admittedly an expensive preference--is to replace rather than repair leaky planter hoses. With a new hose in hand I attach one end of the new hose to an end of the old hose, and use the old hose to pull the new hose into place.
In the March issue of Farm Journal, on the "$100 Ideas" page, Kristin Gall offers a clever way to connect hoses before the pulling process begins. He welds chain links to two pipe fittings, then screws the pipe fittings into the ends of the hoses to create a smooth, strong way to temporarily connect two hoses. Otherwise, it's best to use a combination of mechanic's wire and duct tape to connect the two hoses smoothly enough not to snag on obstructions, but strongly enough to withstand significant pulling stress.
If the decision is made to repair/splice the leaky hose rather than replace it, for gosh sakes be sure to tie a long stretch of strong mechanic's wire or secure an electrician's "snake" so you have some way to pull the hose back through the bar after it's repaired. Be certain to not only make a smooth, snag-free connection where you attach your pulling device to the hose, but make sure any splices or repair connectors used to fix the leak don't have square edges that will catch and snag as you attempt to pull it back into place.
Don't be afraid to liberally lube hoses with something slippery before you attempt to pull them into place. Especially new rubber hydraulic hoses---fresh rubber tends to bind and resist traveling through tight spots.