JD 790

Are the rods that go to the pumps stroking when you move a lever?

OK, That may explain why it not working if it needs air pressure. I'm unfamiliar with this type of system. I searched for a hydraulic schematics that show the air system, but couldn't find one. I just went to look at the machine the other day for the first time, and after 3 hrs of messing with it, with the mechanic, even he didn't know what to look for. The Hydraulic filters would not come out, even with a pry bar. They are loose and bobble around loose, but will not come out, as if they were hung on something, and that's on both filters. I can't remember if the rods are stroking at the pumps or not but it doesn't sound like there delivering full pressure and there was fluid dripping from the pilot pump. There are at least 3 hoses that need changing, and the rotory manifold if leaking, not sure how much it will cost to fix that.

I'm still interested in this machine if I can figure out how it works.. Yeah, if you have a schematic of the hydraulic system with the air system that would be great, that might change my mind if it's worth getting or not.

Those filters should come out easy, maybe deformed on bottom. You have to check the hydraulic oil level with the stick straight out boom down and bucket in full curled position, if the sight gauge isn't to foggy. If the level is to low and your not reading air pressure on gauge that machine will feel dead. All excavators that I'm familiar with have some way of adding pressure to hydraulic tank. I suppose the real early machines didn't do that, not sure when they started to add pressure to tank. Maybe the early machines were gravity fed so not so important. I'll dig out the book for diagram tomorrow.

To bad you don't live closer, my Hitachi 143/JD792 is for sale.

Another thing to check would be the inlet screen if it has one. We had to clean the one on our Hyundai 290 recently, and there was a noticeable change in the hydraulic response.

As Puffie40 suggested, there is a screen inside the bottom of hydro tank.

The JD790 was same as UH09 japan market/UH103 european market. Those Hitachi models weren't allowed in the US because it violated the agreement JD had with Hitachi, not to allow the same size machines as they made for JD into the US. You were able to get them grey market though, as I did just that.

Thank You All for your responses, I have decided to pass on this machine. There's to many issues going on with machine, and by the time I put all the money into it, I could have already bought another machine that runs and works as it should without all problems.

Diminishing returns. I get it. Let us know your next find.