D6-5r Transmission / clutch slipping? Trouble climbing hills question

Hello,

I was hoping that someone could offer some advice on some symptoms I experienced today.

I have about 5 or 6 hours on this d6-5r that has not seemed to have any major issues until some concerns today. 

At first while pushing trees and or hitting immovable objects the tractor seemed to bog down and nearly die until I popped the clutch. It felt very strong in general.

1st noticeable change: The next day I started noticing that while pushing a tree or otherwise bogging down the engine would just kind of bog down but kept on at least idling despite not moving while still in gear. 
2nd issue: Then I noticed an extreme lack of power almost unable to reverse of a moderate incline. This seemed to worsen as the day progressed. I nearly could not backup without using the blade to life the tractor / add blocks under the tracks to back off the incline. 

After checking I found the transmission fluid was about 1.5 inches below full and there was a small leak of transmission fluid under the final drives. 

So my main question is would low transmission fluid explain these issues? Is there a torque converter on these old tractors? Or would only explanation be clutch related? needing adjustment or perhaps new clutch pads?

Thank you in advance for any tips.
 

Clutch is slipping by your description. The more you use it the worse it will get. You can adjust the clutch and the instructions are listed in the "Operation and Maintenance Manual". There is a special tool that helps to undo the lock nut but you can get it with sockets.

Nothing special in the old 5R transmission. No torque convertor.

Either the adjustment is out or the clutch plate has oil on it. 4R and 5Rs' are all dry clutches.Lift the access cover on the clutch and take a look when not running of course.

The low oil in transmission is not the probleme ,check the master cluch and stering cluch adjustement

The clutch dies not like to be slipped. Very hard to doze trees without slipping the clutch. The other thing is all the linkage is exposed and dust gets in it and making feel as it is adjusted to tight. The old timers liked to put a few drops of oil on the linkage. But after a time oil the gathers dust, time to wash or blow it clean and start over.

All gear drive so oil just lubricates all the gears and bearing.

Thank you all, I will wash and inspect the clutch and look up the adjustment procedure in my manual.