I have a kobelco SK 330lc -6 version. I have an issue where my bucket in the middle of the stroke slows down for no explainable reason sometimes. It happens both when you go in and when you go out. Sometimes it goes full speed, sometimes it is slow. We tested to see if the cylinder was the problem putting the arm out and chaining the bucket to a tree, then turning the machine off. We use chain binders and put a ton of force on it and there is no leak by. My other excavator, the John Deere 490E has cylinders that very quickly drift, and this behavior is not the same. I watched the main system pressure as we go back and forth in and out with the bucket, there are no spikes in pressure like what you would see if a pin was warped and dragging or something like that. And the pressure goes all the way up to the system relief pressure around 6000 psi on both bucket out and bucket in when you reach the stop. So I don't think there's anything like the pins or linkages or a bent rod dragging. We tried pulling out what we believe is the main valve for the bucket, it appears to be one 18-in long piece for both in and out. So if the springs or something has failed on this I could see it being the cause. The only other cause I would be aware of that would cause an isolated issue like this on both the bucket in and out could maybe be some sort of problem with the control stick. What do you guys think?
I recommend you try performing a "Bucket Cylinder-Drift Check". I suspect that the piston seals of the bucket cylinder have worn out, leading to that symptom.
Thank you! We tried a version of that in my original post - "We tested to see if the cylinder was the problem putting the arm out and chaining the bucket to a tree, then turning the machine off. We use chain binders and put a ton of force on it and there is no leak by. My other excavator, the John Deere 490E has cylinders that very quickly drift, and this behavior is not the same." I discovered that one of the Nega-Con pressure sensors has gone bad, so I am replacing that with a $25 one off Ebay instead of the $400 one the Kobelco dealer has. We will see how it goes.
Hold buzzer stop and turn the key on to access the diag screens. check your pilot pressure sensors for bkt. they should read 450psi at full joystick movement and 0 when in neutral.