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I am dealing with about the same problem in my 641D. Over the last couple of years, I have thought that the loader was getting slower and weaker and that the 3 point wouldn't lift as much. This Spring I gave the tractor a real workout with a new box blade and ended up with both the loader and 3 point not working very much at all. I also found that my hydraulic oil looked real foamy, almost like a milkshake. It was obviously sucking air someplace. Replacing the oil didn't seem to help much, so I decided that I would overhaul my hydraulic pump. I ordered the Tisco hydraulic pump repair kit and also a new wobble shaft. When I got the pump apart, it looked like the old shaft was pretty grooved where the seal rubbed on it. Unfortunately to replace the seal, it was necessary to remove the cone for the tapered roller bearing. I tried several ways to get it out, but finally ended up grinding it apart with a die grinder. I installed a new roller bearing, but since I didn't have the right tools to either remove or replace the needle bearing, I did not replace that assembly (which I now think was a bad mistake!) I reassembled the pump and put it on the tractor and it seemed to work fine for several hours. Then it seemed to lose lifting power. Over the last month, I have tinkered with that hydraulic pump over and over. Now it is obviously losing prime and pulling air into the system from somewhere. If I open the pipe plug and turn the engine over, it will burp out some air bubbles and hydraulic oil and then work for a few minutes, especially if it is cold. But it doesn't have any lifting power and will hardly lift the empty bucket. After a few minutes, it loses prime and the only way it will work is if I again remove the pipe plug and turn it over with the starter. I have had the hydraulic manifold off over and over. I carefully cleaned it and am fairly certain it does not have a crack that could admit air. I am suspicious of the tube that goes through the clutch area, transmission and into the hydraulic oil chamber. But it doesn't leak hydraulic oil into the clutch chamber and is not sucking the transmission dry. I would have thought that if it had a hole that would leak air in, that hydraulic oil also should leak out of the same hole. I ran a long stiff wire down the intake tube to the hydraulic chamber. There did not appear to be a blockage, and I could see the end of that wire looking into the hydraulic chamber through the PTO shifter hole. I had read that there was supposed to be a screen in there somewhere, but I couldn't see it and don't remember seeing one when I had my top cover off about 8 or 10 years ago. I definitely could see the end of the wire moving around. I ended up having to order them, but I finally got some square cut O rings. They didn't seem to make any difference. The air entering the system has to be coming in somewhere. I have carefully sealed the joints at both ends of the hydraulic manifold with new O rings and also with silicone, after carefully cleaning all surfaces as if to paint. No difference. I have had it apart over and over and over, and I am getting frustrated by the problems. When I had the manifold off, I found that a 1/2" copper water pipe will fit inside the intake tube, and a 1/2 to 3/4 adapter fits inside the end of the tube where the manifold bolts on. I cut some copper pipe to long enough to reach the hydraulic oil tank and soldered the adapter on it. It then slid into the regular hydraulic intake tube quite tightly. I tried using O rings to seal this inside, but it doesn't seem to work any better that way. I have considered using JB Weld to seal that tube inside the Ford intake tube, but so far have not wanted to make the "repair" that permanant, since I really can't say for sure that the tube is leaking. So I have about decided that the pump itself has to be leaking the air where the shaft and seal meet, probably because the shaft is wobbling around because I didn't replace the needle bearing assembly. I guess I will remove it again and have it professionally repaired by the New Holland dealer. I really need that tractor--it is the only one I have chains for, and I probably will need to move some snow in the next couple of months. It has seemed to me that the oil moves through the intake tube slower than it should, and that would maybe cause quite a vacuum. But I can't think of anything else for me to do to make it flow better. If there was a screen or filter on the intake tube, I don't see how I could not have poked big holes through it with the heavy wire. Any ideas? I guess I will be talking to New Holland tomorrow. Hope it will solve my problem. Good luck with yours.
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