Why gearboxes start leaking

There’s rarely a single cause. In the field, you usually have to check several things before you know what you’re actually fixing.

Oil seals wear out

Oil seals are the weak point in most agricultural gearboxes. They sit against rotating shafts, deal with pressure changes and hot oil, and get covered in whatever’s floating in the air.
Run them long enough and the rubber hardens and loses its spring. In a dusty field, grit works between the seal lip and the shaft and chews both of them up.
The usual sign is oil around the input or output shaft. Most people’s first move is to swap the seal, and often that does the job. Not always, though. If the shaft already has a wear groove or the bearing has play, a fresh seal will start weeping again within a few weeks. That’s why anyone who’s done this more than once checks the shaft and bearing before fitting a new seal.
Blocked breathers build pressure
A clogged breather is easy to miss, and it accounts for more leaks than people expect.
The oil inside heats and cools as the machine runs, so the air pressure inside the case keeps changing. The breather is what lets that pressure equalize with the outside.
But fields are full of dust, soil, and damp, and those clog the breather vent over time. With nowhere else to go, the trapped pressure pushes oil out through the nearest seal or gasket. In that case, fitting a new seal without clearing the breather just buys you a few days.

Wrong oil level or wrong oil

Both directions cause trouble. People sometimes overfill, thinking more oil means better protection. It doesn’t. When the box gets hot, that extra oil expands and forces its way past the seals.
Running low is just as bad. Without enough lube, the gears and bearings run hot, wear faster, and the extra heat eventually works the seals loose. Stick to what the manufacturer specifies for that gearbox, and for the conditions you’re actually working in.

Worn bearings and other mechanical faults

Bearings keep the shaft where it belongs. Once they start to go, the shaft wobbles a touch, and that wobble rides on the seal lip and tears it from one side. That’s how you get a leak that was never really about the seal.
Lots of things wear a bearing out early: overloading the machine, a careless install, constant vibration, or just years of hard service. If the box keeps leaking after two seal changes, don’t fit a third one yet. Find out what’s actually moving first, because chasing the seal gets expensive fast.