If you donít mind I would like for you to answer some questions
Did the 3pt stop working all at once or did it happen over time?
[It is possible to bust the upper housing on a 30 series tractor that holds the cylinder for the 3pt if you had a heavy item on your 3pt and it was jarred harshly like crossing a ditch at high speed]
I also believe there is a seal on the end of your cylinder for the 3pt that can leak. Its also possible to have a really bad leak at the cylinder piston.
I think your problem could be the load check valve on the 3PT control valve.
My 930 diesel done about the same thing as you describe and I was able to cut the ball seat with a grinding stone and using a soft drift/punch re-seated a new ball.
I also suggest you do a pressure test at the remotes to see if they are working correctly.
Take a pressure gauge and screw on a male hydraulic tip and test the pressure on the outlet both sides and then let us know what you get.
I know you said you cleaned the 3Pt control valve. Did you change any settings and the adjustment screw on the plunger that rides on the ball that determines when the lift will drop?
[The plunger had what appears to be a bolt with a nut on the end used for locking it in place]
I know this is frustrating but itís a problem that can be fixed.
The worse problem will be a cracked housing the least will be a worn load check valve.
Let us know what you find.