Bobcat builds pressure when off?!

[bheston]

My brother-in-law's Bobcat will build hyraulic pressure when shut off. It will actually lift the front wheels off the ground when sitting overnight.

Is this a faulty hydro filter? Any suggestions?

 

[some zilch]

its actually dumping pressure on the base end of the boom cylinders, and in theory, the weight of the boom/bucket pushing towards the ground, combined with the "counterweight" effect of the rear engine, the relativly light front axle will lift off the ground. i work for a bobcat dealer, and this happens all the time.

basically there is internal hydraulic leakage

 

[bheston]

Any suggestions on where the internal leak would be? Is it the valve to the boom cylinders?

 

[J_J]

Yes, you can build pressure with engine off, for this reason. If a loader arm was in the max up position, the weight of the metal that the cylinder controls, is acting like a pump, and will try and push the fluid somewhere. It will probably push it through the valve until the cylinder has depleted all it's fluid. The lift arms on my Power, if left up at engine shut down, by the next morning , the arms are on the ground. Also, if there is an accumulator in a circuit, you can have some stored hydraulic fluid under pressure, and this could be dangerous if someone activated a circuit just by playing around. Pilot operated load checks will prevent the leak down in most cases. I believe that most loader valves will leak to some degree, some more than others. Otherwise, quality control. .

 

[George G]

I would have too guess, It's one of the models with two hydraulic cylinders for tipping the bucket back and forth. Not one of the models with just one hydraulic cylinder in the middle. Is it safe to assume that ?

 

[bheston]

Yes, it has two cylinders. What do you think is the problem?

 

[J_J]

Yes, it has two cylinders. What do you think is the problem?

Do you know if you have a hydraulic accumulator in your Bobcat?

Just thinking about this situation, that the Bobcat, after shut down, will lift the front wheels off the ground overnight seems almost impossible except for maybe two reasons, With the bucket on the ground and engine shut off, there is no force that will cause cylinder action except heat build up, and I suspect that overnight, that won't happen. The only other possibility is, a pressure vessel like an accumulator that stores hydraulic fluid under pressure. If this fluid leaks through certain valves, then it might cause a cylinder to lift. Now the accumulators are usually small, but if you had one in the Bobcat that held as much fluid as a cylinder, then if it leaked out through the hydraulic circuits, to the lift cylinder, then maybe. It would also have to select the correct valve, and pressure side of the cylinder to cause any lifting action. I can think of no other reason that a hydraulic cylinder will move with out some help.

Suppose you leave a bucket in the air, the pressure on the cylinder might cause the fluid to go somewhere, and most of us have had that happen, but when the bucket hits the ground, all fluid stops flowing, and all movement should stop. The lift arms on my PT, have never lifted the wheels.

 

[George G]

I would say, their is a small leak in one, maybe both, of the piston seals in the cylinders that run the bucket. When you shut down the Bobcat, you leave some pressure(maybe 300-400psi) in the bucket cylinders. Thats enough too start the action. Next time, after you shut it down, move the controls for the loader and bucket back and forth a couple times to relieve all the psi in the cylinders. See if it still dose it. Let us know if that helps.