You sure have my interest peaked to see if a answer is found to how this happened.

??? Possibly a oil quality issue?

Not being familiar with the grader is oil for the clutch shared with the engine?


Related ................maybe.................maybe not.
I grow up around the old Be Ge hydraulic pumps on Cats. As one guy put it anything less abrasive than gravel worked as fluid in them. With cost of Be Ge parts and modern tillage implements liking more pressure to lift, I have cobbled several pump and controls together. Using cheap Surplus Center pumps and control valves.

The disc stopped opening and closing one day, on investigation I had a wax build up on the suctions screen. As it was a cheap build I used cheap oil. The yellow bucket ( generic tractor store brand) tractor transmission hydraulic fluid. Was it mixing of brands or a Chinese viscosity enhancer problem. The good news pump was not damaged. But not at all what I expected to find.

I have an 80C series No. 12 grader that I bought last year with an oil clutch. My clutch would not release when I purchased the grader but stopped releasing.  Last summer, I pulled the engine and found that the friction discs were stuck to the pressure plate/flywheel.  I cleaned the pressure plate and flywheel and replaced the friction discs, bearings, and the clutch brake.  The clutch still would not disengage... I pulled the engine again thinking that I did had not installed something properly. After pulling the engine a second time and confirming that all of the parts were installed correctly and the pressure plate was functioning; the clutch still would not disengage.
After much frustration, I found that I am not getting enough travel on the clutch release rod.  If I set the pressure plate clearance to specification (1/8"), the clutch will not release.  If I remove the clearance such that the pressure plate is partially depressed then I can get the clutch to release to allow gear changes but the clutch slips under any modest load.  I can't find a "sweet" spot in the adjustment that allows for disengagement without slipage.  All this was last fall and the machine has remained untouched since then.
I am out of conventional ideas so what I am planning on doing which goes against everything that I think is sacred... I am going to fabricate a longer actuating lever that connects the clutch pedal rod to the clutch actuating rod to give me more travel and see what happens.

Did you check the clutch forks for excessive wear?

Yes; the clutch forks looked fine and any wear would be negated in the adjustment process.
Thanks for the thought and question.

Deita Kj, Did you ever find out what actually caused the friction disk to stick to the flywheel and pressure plate? Effective with 80C1 and 8T14782 the free movement of 1 7/8 to 2 inches at the top of the pedal movement by adjusting the clearance between the clutch levers and the release bearing thrust ring is what the operator /maintenance instruction manual calls for. This 8T20003 had the 3 inches of movement that was called for with the earlier models. After adjusting the free movement I noticed a lot more travel of the pressure plate, but with the friction disks stuck as tight as they were it made no difference. if you haven't already made that adjustment it might give a little more travel on the pressure plate. You might even take all the free play out of the pedal to just see if it will give enough travel to release. Just a thought bboaz

bboaz; The friction discs were stuck to the flywheel and pressure plate.  When I removed them it sounded like pulling apart velco.  The chemical makeup of the friction discs fused to the surfaces.  I confirmed that oil was being slung within the clutch compartment so the problem was not a lack of oil.
The first photograph is the before cleanup and after cleanup of what the pressure plate, flywheel, and intermediate disc.
 
The following photograph is the old and new friction disc
 

Deita Kj, Exactly like this 8T20003. Maybe this has happened more than we realize. Thanks bboaz

bboaz; Some folks confuse the master oil clutch on these graders with the oil clutches on crawler tractors.  Many find it hard to believe that these friction discs can stick to the plates because it rarely (maybe never) happens with crawlers with oil clutches.  These grader clutches function similar to car and truck clutches and are a completely different design from crawler clutches albeit both termed "Oil clutch".
What would be very helpful to me is after your have repaired, adjusted and confirmed that your clutch is functional; measure the distance that the clutch control rod moves when the pedal is fully depressed.  Before I fabricate a new control arm I would like to know what the "normal" control rod travel distance should be.
 

If it's going to set awhile block the clutch pedal down so the discs dont have pressure on them.

Delta Kj, I can do that. got the new friction disk today and put the clutch back together. Waiting for a new gasket for the clutch housing.
Rome K/G, If it's going to set awhile block the clutch pedal down so the discs don't have pressure on them. What 's that going to do the the springs in the pressure plate. I would think that after time it would weaken the springs. If it were an over center clutch like in a dozer, just leave the clutch disengaged. There are no springs to compress as there are in this automotive style clutch. Maybe half way just to take some pressure off?????????????