Gear box filling with hydraulic fluid. All the hydraulics are still functioning. Is the shaft seal on one of the pumps causing this?
Which gear box are we talking about? The chain cases for travel or the flywheel housing? If it's the chain cases, your shaft seal from the motor is probably failed. Has the return filter ever been serviced? Does it have a vented hydraulics tank cap? Are any hoses smashed ir pinched under the cab?
The engine gearbox. The manual doesn't mention a return filter where would it be located? It does have a vented cap I cleaned it by blowing air thru it.
So, is your pump direct drive off the flywheel housing? No in-between open cavities? Any other equipment pumps running off the same housing?
Yes direct drive off the flywheel. No other pumps
Then, it looks like your pump assembly has failed. The input shaft us allowing oil to slip past. Again, this could be caused by faulty hoses, plugged filters or dirty cap. Check all your mounting fasteners, motor mount insulators etc. Make sure the pump stack isn't flopping around in there. That could be causing undo strain on the input shaft. You're going to have to remove it for repair, but while you're doing that, search for the root cause. Good luck
I'll check everything you mentioned. Thanks for your time.
BTW, swing by new member introductions and introduce yourself, let everyone know what you're up to. Lots of folks here willing to offer suggestions or personal experiences