Well, I'm quite familiar with your fluid level problems.
Transmission oil is transferring to the bevel gear case. Sounds like yours is doing it at an alarming rate.
Mine was doing it at a decent rate, and it seems to be a common issue with worn out old 955s. I went and looked at a 955 for a friend two summers ago and it was doing the same thing.
Check out the last post in my thread here:
https://www.heavyequipmentforums.co...d-from-transmission-to-bevel-gear-case.83596/
Some general knowledge: the 955L engine is coupled to the transmission via a shaft with flexible coupling. The transmission is directly coupled and bolted to the bevel gear case. The bevel gear case is your "rear differential" and contains your brakes and clutches. On each side of the bevel gear case are the final drives. The brakes are mechanically manipulated, the clutches have hydraulic boosters which are activated by the pedal linkage. So you have pressurized hydraulics in the transmission and in the bevel gear case.
From the view of sitting in the operator seat, the hydraulic pump is located on the front left upper area of the transmission. Going down the left side of the transmission you have the transmission screen and transmission filter. Behind and below those are the transmission pump and PIGGYBACKED on it is the steering clutch pump. This is where the problem lies.
The transmission pump drives the steering pump with a common shaft, and when things get worn the transmission pump will transfer fluid into the steering pump.
First you should remove, inspect, and clean the steering gear screen. It is located on top of the bevel gear housing, left side, low and behind the transmission. Not fun to deal with. If this is plugged up it could be increasing the transfer of fluid. Also, replace the steering clutch filter that is located directly under the seat on top of the bevel gear housing.
Also, replace the transmission filter and the clean the transmission screen.
Now, if you can slow that fluid transfer, you can do another trick I did for awhile. That would be manually moving the fluid from the gear case back to the transmission. I did this with a cordless drill driving a fluid transfer gear pump. Slide the suction hose down the filler tube for the bevel case and pump the fluid into the transmission. Go work the machine awhile, then repeat as needed. Mark the suction hose so it is inserted down the fill tube to the proper level. That way when it stops sucking the level is correct.
If you want to fix the problem, you're going to need to replace the transmission pump, maybe the steering pump, along with a bunch of o-rings and seals you will encounter. I went aftermarket on the transmission pump to save money and so far I have not regretted it.
Most likely, you will find your lack of transmission response is also solved when you replace the pump, as I did. Your transmission is dependent on hydraulic pressure to function properly. Mine was also sluggish or non-responsive at times.
You will need to drop the main pan under the transmission and work on this from above and from below. I've worked on a lot of stuff, and this was one of the hardest jobs I've ever tackled. The parts you're working on aren't big, but they are just insanely difficult to get to. I also removed a lot of lines that were leaking at connections and replaced seals to reduce leaks. You won't get them all, but you can get a lot of leaks fixed with just an o-ring or gasket. One of my major leaks was the transmission input shaft, so give that a close look. That repair was a bit more involved.
You will need the correct part book for your machine to look up seals and other parts.
The service manual will also be handy to have. Again, it needs to be the right one for your machine.
Regarding your stuff control lever, they should feel the same. A mechanical linkage joint is probably in need of attention. You can try spraying the connections with Kroil, but I replaced a lot of my rod ends and disassembled/cleaned/lubricated the pivot points.
Excess slop in the transmission linkage can also contribute to the machine not going into gear. Those pivot points and rod ends were in really bad shape on my machine and caused me some shifting issues. The pivot points on top of the transmission, one at the front of the top cover plate and one at the back, are critical and needed the most attention. If the front one has a lot of play in the shaft where it enters the top of the transmission, I "fixed" this by welding a heavy washer to the top of the housing around the shaft to hold it in place, as replacing the internal bushing is next level.
So with the floor pans removed, move the linkage around and see what has slop. Your service manual will give linkage adjustment measurements too.
Add this to your maintenance to do list too: adjust the pedal linkage and brakes per the manual. This will make sure your clutches and brakes are working as intended.
I was in your exact same shoes not long ago, in terms of knowledge and experience. Between forums and the manuals I got it done. My 955L has thrown other repair challenges at me, this past fall was really interesting, but the machine lives on. And my significant other is getting her dream horse farm built slowly but surely.
. After another trip into town I finally came home with some TO-2 spec oil. 
