I have a 70's John Deere 300 Industrial tractor/loader (similar to a 1020) with a JD9250 backhoe attachment which has served me well for 15 years of occasional use. Recently, it has developed a hydraulic problem. First, after using it for hours, suddenly the loader and backhoe would not operate at all. Engine runs fine, transmission fine, tractor moves fine. No funny noises, just no movement on the loader or backhoe. After about 10 minutes, hydraulic function came back, then came and went intermittently.
The next day, tractor ran fine for about 30 minutes, then the same intermittent loss of hydraulics. Next day, same thing. Oil level is fine.
I thought it might be clogged filters, so I changed both hydraulic filters (the transmission one and the loader return one. I also changed the hyd. fluid. The oil fluid and filters were clean as new. So I started it up, let it idle to self-purge for about 20 minutes, and it operated fine for about 5 minutes then, intermittently began to lose hydraulics, with occasional shuddering and air banging around in the lines. No amount to extending/retracting of the cylinders would clear them out. I have changed fluids on this tractor before and never had this problem. It feels like air is being sucked into the lines somewhere, but hoses and lines look tight.
Sometimes the machine runs normally. It has plenty of hydraulic power, plenty of bucket lift. It can easily lift the tractor with down pressure with either the front loader or backhoe. It can run this way for minutes. When it is running normally, it sounds normal. This makes me think the pump is okay.
Then, suddenly, no pressure. If a bucket happens to be up, it stays up until you put it down. Then nothing. At this point, it still sounds normal, no click-click-click like it is out of oil. If you hold a valve open, no pressure, no funny noises. It can be like this for minutes at a time. RPM doesn't help. Then, sometimes, the cylinder will start to move, slowly as it builds up pressure. Sometimes the whole tractor will shutter loudly as the cylinder ratchets up. You can hear air pinging around in the lines. Sometimes, after shuddering for a few seconds, the pump will "catch" and pressure and operation will be normal for a minute, or so. Then, it goes off-line again.
The problem seems to more related to the loader than the backhoe. In fact, when the loader cylinders are "dead", if you open the backhoe valves and move the backhoe pistons a bit, you can restore pressure to the loader for a while. The tractor itself always has moved forward and back on its own.
It does seem like an air suction problem, but I am not sure. I am not very familiar with hydraulics and would be grateful for a point in the right direction to troubleshoot this problem.
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