stiff 3 point lever

well I continue to work through the rebuild and resurrection of a 1979 AC 5050 4wd tractor. I am getting near the end. This tractor had poor maintenance except for oil and air filter changes but it has low hours and the motor seems perfect, easy starting, no smoke or blowby. 8 spd tranny seems sound as is the 4wd. My last and potentially serious problem is this: the 3 point functions properly but is very stiff and hard to move. It has an inner cork gasket but I can see a tiny gap so  I don't think its binding as I took off the nut. One issue is that the transaxle/hydraulic fluid was milky white with water and there was NO hydraulic filter(ORDERED!). Naturally I drained and flushed it.What could be causing this? binding in a valve caused by corrosion or debris? Is there  an easy fix ? I can't get the lever assembly off the keyed shaft even with a puller.

Thank you. That is actually my plan. I am waiting for the filter before I run it. I have  just received a service manual which shows the function and disassembly. The magnetic plug was LOADED with particles, like an inch thick. yikes. I assume the spook is binding because of debris  and I can hope that flushing and filtering and moving might improve the stiffness but I have a nagging fear it wont. O would like to know where in the system the binding is occurring.

If it were mine I would use 50-50 hyd. oil and diesel mix because it sounds like a lot of sludge and crud in bottom and would probably do that a couple time after couple hours use each time. Leon

THANK YOU GENTLEMEN FOR YOUR SUGGESTIONS. As to the half and half mothers milk in the tranny  I think I will take the suggestion of diesel/hyd fluid transfusion for a quick use and flush after the filter arrives. The alternative to keep putting in 20-21 qts of all trans fluid is pricey. The three point lever problem is fixed. I was able to isolate the external lever control from the hydraulic control valve lever. The hyd lever moved freely. The problem was in the external lever. I pressure washed and cleaned and scraped all the rust and cow manure off the control bolt which had cork and felt gaskets and kroll/WD/PB and synthetic teflon sprayed the bejezus out of it and VOILA!. It moves freely again. Big relief. The control valve replacement is now on hold permanently I hope. The painting is almost done,the new seat arrives soon as well as  new India made 8 ply R1 tires. all that is left is new brakes and trying to find fenders and a ROPS which is eluding me so far. I like the design and construction of this heavy  4WD 55 hp FIAT tractor which seems overbuilt and well designed far superior to the more flimsy  2WD Massey which it replaced.  I need it for haying next week because my little 35 hp Kubota really struggles running our New Holland 270  baler.  This AC5050 looks like it was designed by mechanics rather than front office stylists and strong well designed tractors like this should be saved and not scrapped.