Hydraulic leak from top of transmission/rear axle housing

[DigitalDruid]

Newbie to tractors here, but I've had decades of mechanical experience...
I have a L2501DT tractor that has a hydraulic fluid leak that appears to be coming from a bolt on the upper left side of the rear housing cover plate (differential/axle) under the seat.
In the pictures you can see the leakage from the top of the housing around the bolt, down the left side of the housing and all over the drawbar (removed for cleaning).
It may be due to me overfilling the fluid as I find it extremely difficult to see the fluid level through the inspection port, which just looks cloudy all the time. I suspect that as I used the tractor, the fluid heated up and expanded, creating too much pressure inside the rear axle/transmission housing...
So I have three questions for the more experienced folks out there:

1. Have you seen anything like this before?
2. There appears to be some sort of flexible washer under the bolt head. I'm currently assuming if I remove the bolt and replace the washer, that will stop the leak (provided I can get the correct fluid level in the hydraulic/transmission/axle system).
3. Is there any OEM or 3rd party replacement part for the inspection port that offers better visibility?

Thanks in advance for any assistance you may be able to provide.

 

[DigitalDruid]

From other sources, I now suspect I've definitely overfilled the case...as they pointed out the vent tube (a little black U-shaped hose) exits right in front of the bolt head that I originally thought was the source of the leak.
Another source said they used a bit of John Deere red hydraulic dye to aid in seeing the fluid level through the inspection port.
So I've got an errand to do and a procedure to slowly drain the fluid (while on level ground) to get it down to the correct level.
Any additional comments here are appreciated...since I'm learning. TIA.

 

[je1279]

From other sources, I now suspect I've definitely overfilled the case...as they pointed out the vent tube (a little black U-shaped hose) exits right in front of the bolt head that I originally thought was the source of the leak.
Another source said they used a bit of John Deere red hydraulic dye to aid in seeing the fluid level through the inspection port.
So I've got an errand to do and a procedure to slowly drain the fluid (while on level ground) to get it down to the correct level.
Any additional comments here are appreciated...since I'm learning. TIA.

On level ground, I would just suck out some of the fluid until you see a small area at the top of the sight glass without fluid. While your there, make sure that the hole at the end of the vent tube doesn't have any crud buildup in it. After that, you should be good to go.

 

[North Idaho Wolfman]

You nailed your issue.
Overfilling is a bad thing.