Do you think the cylinder was filled with water?
That in it's self would be very very odd.
I doubt completely filled. It's a 70 year old set of Oliver transport disc. A hose was bad on it and I replaced it before hooking up to it. The owner uses it for food plots off a Ford 5000. Both pieces sit outside all year and only get used once late summer. I doubt the Ford tractor has been serviced in the last 10 - 15 years after being bought at auction.
My tractor I have completely drained the hydraulic oil and changed both filters in the last 3 years - 2x after 2 hydraulic hoses failed. The power hose form top side of transmission under the seat up to the loader valve and one to the back. And both times I had rear tire off for better access for hose replacement and at the filters straight on. Both times it was warmer outside changing oil.
My tractor is 20 years old/ 1345 hours and has sat outside unfortunately the whole time in upstate NY.
This tractor since new would start easily at 10-15F with just glow plugs and still does. Some years if we had prolonged cold below that I would plug in block heater to make it easier starting on battery and engine oil.
There were years when I traveled for work away during winter and my daughter or wife would use it plowing. Originally the tractor was used round baling when we had sheep & beef and I was home.
Now I used it for skidding saw / firewood logs, plowing and moving buckets of firewood to the house. Doesn't get started every day.
Reason for low hours is I was away traveling for work my last 10 years before retiring.
I did have some gunk come out of the drain plug that is within the bracket that supports the rear of the fuel tank. I don't think that's the lowest point drain either. I did capture it in a clean container to look at later.
After reading another M6800 hydraulic post here - I've never removed steel lines under hydraulic pump and blew them back.
Being 67 it's not easy sliding under the tractor to get to the hydraulic filters and there location near tire just traps dirt on top of them.
This weekend it will get cold again down near zero so it will be a test if system is cleaned out enough.