It has been a long time since I worked with Case. I did some transmission work but, not allot. The first place I would check is the filters. Do they have debris in them? Can you check the clutch pressures?
The only odd thing that might be happening is if it was locking three clutches at the same time. Does machine stay still in neutral? If it does the forward clutch could be damaged.
Let me know if I can help answer any questions will the test results.
Your right, I was think about the 850, it has a single clutch.
If I remember correctly, the brake housings are on the front of the transmission. Do these seem to be getting hot? I remember the parking brake linkage would stick on some and in forward this could drag the transmission to a stop.
Inspect the park brake lock and linkage. Spray all the levers on top to free-up the pivots points. This was more common on the 850 so, I am guessing here.
The filters will be a good start to find out if you have damage. Cut them open and lay them out to do a detailed inspection. Save some oil in a clear container and look at it through a light.
Let me know if I can help you any, I will be glad to try.
Yes metal shavings, brass colored shavings and fiber from the clutches. If it is old, it may have bronze clutch disc. I only have seen the paper fiber discs. Most of it will be very fine, about the size of sand. Pass a magnet over it to see how much iron is in it. The more iron the more likely to have carrier or housings damaged. This would mean an expensive repair. Any large parts would be broken damage, like clutch housing.
When you said reverse is fine, have you traveled more than one turn of the track? Just want to be sure the rails aren't seized.
This transmission need to be laid on it end to have to bearing setups and that means it will have to be removed from the frame. That is a huge job if the bolts are seized. Cutting the bolts can be very difficult.