Thanks for the reply.
The hoe is new-ish... it was new in July and now has 8-10 hours of digging on it. I have already done the "purge the air" procedure with no difference.
The dealer has also worked on the backhoe twice already: the first problem was it would not hold it's position and would droop. He said the fix was "cleaned some assembly gunk out of the relief valve". Second problem was the dipperstick had no power and moved VERY slowly. He said the fix for that was "replaced a valve".
The dealer has not responded to this last problem. So I'm hoping to learn as much about it as possible so I can "make my case" to him.
-Jeff
Couple comments. Obviously the hoe has issues. Equally obviously, the dealers is about as incompetent as they come. You probably figured that out, but I thought you might like to hear it from someone else.
Looking at the video it looked like the dipper stick cylinder "sprung back" when tension was removed. If it's fluid bypassing, that CAN'T happen, as in impossible. Fluid doesn't compress - air compresses. You still have air in the cylinder because the cylinder bounced back out. (unless the hose was swelling like 3x it's diameter....). Time to get "aggressive". Pull back that lever and hold it full back
for 1 full minute with the engine at PTO RPM's. Use a watch. Then push it full forward for 1 full minute. You will be bypassing the fluid across the relief valve the whole time and the fluid will be heating up -- keep an eye on that. Actually, a hand. The fluid will get hot, that's ok as long as it stays under 300F. Normal operating temps are 140-180F. Short excursions to 200-250 are not an issue at all.
I suspect that there are multiple issues.
You REALLY need to spend 20-30 bucks and buy a pressure gauge that reads up to 3500 or so and the adapters to fit to the working ports of the hoe control valve. Check that you are actually reaching the operational spec pressure. Some systems set the pressure at the control valve, but some Kubota's set it in a block on the tractor by changing shims. You can get them pretty cheap. IIRC they are like 150psi per mm. On new systems going 150-200 psi over spec is ok as the relief spring will quickly weaken and settle down that much -- then hold it there for a long while. You might have to take off a hose and bring it to a hydraulic shop to get the correct adapter - or else visit the dealer. But I don't have much faith that your dealer can find his butt with both hands...
If you have a cylinder that is bypassing, you can test for that pretty easy (but not neatly!). Fully extend the cylinder, then REMOVE the hose from the cylinder on the retract side. Then with the hose still off, move the control to extend the cylinder. You will probably get a shot of fluid out as the cylinder moves a bit, then you should get just a few drips to a very minor dribble. That dribble will get worse over time as the cylinder and seals wear. Seals can hold in one direction and leak in the other, so you have to repeat with the cylinder retracted.
If there is no air in the cylinder and the cylinder is not internally bypassing (and visually not externally from your vid), then the only place left is the control valve. If the relief valve is set correctly and not opening early (detected by a low gauge pressure) then there may be internal bypassing in the valve.
Hope this helps.