Did you bleed the hydraulic pump by removing the pipe plug in the top of the hydraulic pump, starting the engine or just turning it over until the bubbles stop coming out of the hole you opened? Then you stop the engine, replace and tighten the pipe plug, and you SHOULD have pressure when you start the engine again.
If you have done that and have oil moving through the hydraulic pump, you may have a leak somewhere in the top cover assembly, or possibly a stuck valve. You might want to try removing the cover that has the PTO lever through it, to see what the oil is doing inside the hydraulic chamber. If it is spraying all over at the top, something is definitely wrong. If it is spraying from the top cover cylinder, you probably need to reseal the piston.
The top cover assembly is heavy. The first time I removed one, I just muscled it off, which I do not recommend. A cherry picker or hoist works a lot better and in my opinion is much safer.
Do you have a manual? The procedure for working on the top cover is pretty well spelled out, and it is not that hard to reseal the piston and make sure the unloader valve is free. Or you could take the loose top cover to a tractor mechanic and have the work done, and the unit adjusted (assuming that the tractor mechanic knows what he is doing with old Fords!).
The top cover then needs to be installed back on the tractor, and again, a cherry picker or hoist makes the job a lot easier. You want to make absolutely sure you get the RIGHT O rings and gasket installed properly before you tighten down the bolts, or it will probably leak all over your right leg.
I also have a 641D, with a huge amount of hours on it. It starts and runs pretty well, but has lots of blow-by and leaks some oil. I don"t think they made very many 641D"s--I have only seen a couple of them mentioned on this board over the years. If I ever need to rebuild an enigne, I will build a 172, since it appears that no one sells a rebuild kit for the 144. But as long as my old 144 keeps working, I will keep using it. Hope you can figure out what is wrong with yours...Good luck!