Yanmar excavator blade won't stay down

2004 Yanmar vio-50-2. When I put the blade down to stabilize for digging in about 30 seconds it has come up and I have to push it down again. I checked the cylinder for bypass and there is none. (I disconnected and capped the extend hose and when I then tried to retract the cylinder it did not move. If there was bypass, the cylinder would have extended with this test.) So my problem is elsewhere. Sorry, but I've never worked on control valves before, and I'm not a mechanic, so could somebody explain in plain terms where I should go from here? Is there a likely cause? Do I check the control valve that is manipulated with the handle that is moved to raise or lower the blade? If so, what would be the procedure? There is also a large block of valves back beside the hydraulic pump. I've attached the hydraulic schematic from my service manual, and it shows A8 and B8 as the valves that work the blade. Do I start there? Knowledgeable input appreciated.

Good day & welcome to the forum
Your post eliminates a cylinder as to the cause of the internal leakage issue, you have three possibilities left in the circuit if we take those in order of connection working from the cylinder it looks like this 1/ Central joint 2/ Port relief valve 3/ Main valve spool.
I suggest taking the main line from the positive side of the blade cylinder at the main valve. First raise the machine using the equipment leaving the undercarriage raised supported by the equipment, lower the blade. Stop the machine, remove the line at the main valve and plug the open lines with steel stoppers.
Start the machine, raise the equipment so that the cylinder becomes pressurized if it holds No1. Its good if it still sinks that is where the leakage is at.
No2. You can normally switch the port reliefs from side to side, I cannot be sure how it is on your machine as I am short of the hydraulic schematic,
If this fails it leaves the spool damage requires replacement, Be sure if this has a cable operating the spool that it is returning to the closed position.
If you would like illustrations, I would ask you to share any manuals you have.
Kind regards
Uffex

Why can't it be a failure to return to center of the blade control lever/ joystick?

Thanks for your reply, uffex. Pardon my lack of knowledge here, I need some clarification.
Is the positive side of the cylinder the extension side? So that would be BLD in the attached schematic. Following it through it attaches to the valve block at A8 after passing through the swivel joint. Is that where you are talking about? Disconnect at A8 and cap the ends?
Then I'm not sure what you are describing to test it. Jack the machine up and lower the blade, stop the machine, remove A8 and cap the ends, then start the machine, remove the jack to put pressure on the blade cylinder. If it sags, then the swivel joint is the problem. I have this right?
No2: where is the port relief valve? I don't see that in the diagram. Is it part of the control valve block?
Great idea, I've attached the hydraulics chapters of the service manual for the machine.

heymccall said:

Why can't it be a failure to return to center of the blade control lever/ joystick?

Click to expand...

I'll check that as well.

Good day
I have studied the information you kindly provided, a point that I found unusual is that there is no provision for a port relief valve for the positive side of the cylinder yet one has been installed on the negative side. The remaining questions I have attempted to answer in the attached.
Kind regards
Uffex

uffex said:

Good day
I have studied the information you kindly provided, a point that I found unusual is that there is no provision for a port relief valve for the positive side of the cylinder yet one has been installed on the negative side. The remaining questions I have attempted to answer in the attached.
Kind regards
Uffex

Click to expand...

Thank you very much. I will dig into it and let you know how it goes. Cheers!

Ok. Got sidetracked with something else and finally had time to tear into this today. Disconnecting the control cable made no difference. I ran the engine, put the blade down, and supported the machine on jacks so the blade wasn't holding it. I disconnected B8 and plug/capped the hose/valve. I started the engine and removed the jacks and the blade still sags. So that means the swivel joint is the problem, correct? Any way to confirm that? Removing that thing is going to be a bear.

I also think I noticed an inconsistency in my service manual. If you look at pg 5-3-12, in the diagram BLD (blade down) goes to 'C' at the swivel joint, following C out of the swivel joint it connects to A8, not B8. But looking at 6-2-22, it states that blade down pressurizes B8, which I suspect is the correct one. Am I right this is inconsistent? Any other testing I could do to clear this up?

I think I've cleared it up myself. I still have B8 capped. I started the machine and moved the blade lever forward (commanding blade down.). The engine strained but the blade didn't budge, not even a tiny bit. When I pull the lever back to lift, it did lift a fraction, as the negative side of the cylinder is still attached to the control valve. So B8 is definitely the blade down port, and because I still have blade sag with B8 capped, the diagnosis is the swivel joint. Can you please confirm my thinking is correct? Don't want to take that thing out of there for no reason! Thanks.

Good day Jr
Yes I can confirm that would be the case, if you choose you could run a pressure test please see attached, this is best practice to measure internal leakage.
Kind regards
Uffex