Pressure relief valve

Most Chinese tractors do not have a hydraulic relief valve, but if you have a loader on it, it may have a relief valve on it.

Would have replied earlier but could not get logged in.

If you have a front end loader it is highly likely there is a pressure relief in the control valve.

If you have power steering, there is likely to be a priority valve that may also have a relief valve.

I don’t think a hydraulic system can operate without one, otherwise the oil would just flow back to the tank and not cause any work to be done. Something has to cause pressure to build up in order to exert pressure on a cylinder somewhere else. Think of trying to blow up a balloon with a hole in the end of it.

If your are getting air into the fluid that will definitely degrade performance. Entrained air acts as little springs in the fluid and absorbs all of the pressure put out by the pump.

I would suck some fluid out of the tank and put it in a clear container. Check color and viscosity.  Then run the tractor until the hydraulics fail then pull another sample to compare with the first. That should give you lots of information. Watch the second sample to see if the bubbles rise to the top and dissipate.

A seal that leaks fluid is not likely to be the source of incoming air. That can only happen on the suction side of the the system.

I am not familiar with your particular tractor but I take it to be probably made by Jinma and similar to my 284. In that case the hydraulic tank is below the seat and the 3 point piston is inside the tank. The suction line probably comes out of the tank at the front left corner and has a hard line leading forward to the pump. That is where you need to concentrate looking for a suction leak. On my tractor the tank screen is accessed by removing the suction pipe from a cast housing that is held to the tank with 3 bolts. Inside you will find a short pipe with a screen wrapped around a frame that sits near the bottom of the tank. The pipe treads into the cast iron housing from the inside. If that pipe is loose it could be sucking air there.

If there is an air leak elsewhere in the suction pipe/hose leading to the pump it is likely that it would seep fluid while the tractor sits and thus would leave a visible mark, even if it is only a collection of oily dirt.

Taking the suction screen out for inspection and cleaning is always a good idea and it will allow you to check the pickup tube to housing connection. There should be an o-ring in there, and seals at the top and bottom of the screen.

Do you have a parts manual that shows this break down? If not I can scan that page of my manual and post it on my web page so you can see it.

I do not know about your tractor but my 2006 ft504a foton has reinforced rubber hoses on the suction side, I have not had a issue with mine but that would be a good place for a suction side air leak.