It is an early seventies jd 450 or 450b. The tractor just gradually got to the point where after about an hour of operation it was very slow to engage in reverse and back up. I decided to check the pressures and adjust everything as described in the manual. I only had about 70 psi on the gauge but adjusted the clutch linkage rod to the pump and it would rise in pressure about 10 lbs before dropping of as it is supposed to as the pedal is depressed farther. I then took off the plug for the pressure regulating valve and added two more dimes as shims to the one already installed and was able to raise the pressure to 120 psi. (still not near the 170 to 180 required and added more than two dimes did not increase pressure anymore.) I tried out the machine again and shortly it got worse. I then pulled off the top cover looking for something obvious and I decided to remove the clutch accumulator valve. I found that the line from the pump to this valve would slide back and forth even when the tubing nuts were tight. After removing this pump feed tube which originally had some kind of rubber o ring seal built into it with the ends brazed on I thought this was the problem. I bought some adapters and replaced these fittings with hydraulic compression fittings. I did not go any farther with the valve and put the tractor back together but this did not fix the problem and I could now get about 25 lbs on the gauge. The tractor will move itself and thats about it.
Today I decided to remove the top cover again and remove the clutch accumulator valve again and disassemble it this time and also look to see that the clutch rod is not adjusted wrong and just dumping all of the oil back into the reservoir. I pulled off the valve and in one port there was a cylindrical piece of brass tubing wedged sideways in the hole blocking off most of the passage. I could not pry it up and after taping gently on it it fell down to the bottom. I tried to lift it out with a wire and grease but it disappeared down deeper. I pulled out the clutch bypass spool and was able to blow it into the bottom of this larger port with compressed air. I then used the wire and grease to lift it out but after it just came up it fell off back the clutch dump spool bore and on into the reservoir. I then disassembled the valve, everything looks good, but the screen behind the o ring on the accumulator cover is not there. I believe this is what was stuck in the hole.
Tomorrow I will put this back together and try it again without a screen. I am hoping that this was blocking the hole enough to have caused the problem. I will find out tomorrow and report back. Thanks for asking.