komatsu d21a-5 hydraulic problem

[jeff-pa]

hello, my machine has developed a problem suddenly where the blade will no longer stay in the air in the center hold position with
the machine running, drops in a few seconds. the blade will hold the machine off the ground when pushed down so the problem is
only in one direction.
i suspect the problem is in the control valve as i can hear fluid bypassing in the center hold position. i have rebuilt the lift
spool with new orings, u-seals and dust seal but the result was no change.
does anyone have an idea on where to check to solve the problem?
thanks

 

[AJ.]

I cannot remember what type of seal is on the piston in the cylinders, looking at the parts drawing it shows two packing seals on the piston, the two seals are the same, one goes to the bottom of the piston and the other the rod side, if those seals are directional and if one of them would fail in one cylinder it could cause the problem you are having, it is seldom for that type of problem to be the valve and as you had it apart and the spool is ok it is more than likely in one of the cylinders, it would have been the cylinders I would have suspected first, the seals are wearing parts.
AJ

 

[jeff-pa]

thanks for the reply. After doing alot of research on dual acting cylinders i think that i understand that internal seal failure
would not cause this type of failure. when the spool valve is working correctly it blocks flow to and from the cylinder in the
center position so the pressure equalizes inside the cylinder regardless of the seal. i watched the video where a seal was removed
and cylinder reassembled with no seal and a load will hold as long as the valve works because the fluid has no where to go. neither
lift cylinder has any external leak and the lines do not have a leak. i have rebuilt at least one of the lift cylinders within the
last year due to an external leak on the rod. i think that an internal bore cylinder seal failure might not allow a cylinder to
have force on extend or retract. i don't have a good way to isolate the components.

 

[AJ.]

If you had a cylinder apart you know what type seals are on the piston, with two seals on the piston being the same if they are directional ie one faces one way and the other the opposite way then if one seal should fail that would cause the oil to bypass the piston and gravity will do the rest, did you find anything in the filter, if the cylinder was full of oil both sides of the piston, if it was in the middle of the cylinder with both oil ports blocked off, if the seals are directional and the seal for retracting the rod has failed it could be pulled out but not pushed back in, out of the sixty years I have being in the repair business it was very seldom to have trouble in the valve unless some metal got loose in the system, why did you suspect the valve first and not the wearing parts, you must have a spare kidney to sell if you want to replace the valve.
AJ

 

[WilBury]

Check out this post for a lively discussion on the subject...

https://forums.yesterdaystractors.com/viewtopic.php?t=1560841&highlight=

 

[Jimever11]

Mine had a similar problem. It was a check ball retainer that was broken. This allowed the ball to fall inside the valve. So I had no check valve

 

[jeff-pa]

Good question. When this problem started, i looked at the spools. Only the lift spool had a little fluid getting past the dust
seal so i thought just maybe enough leakage in the spool was allowing the drift. the spool also has directional seals which could
have worn. i did not find any fluid leakage at the bottom of the spool where the spring is. the spool rebuild did not correct
anything. I am waiting for a new filter to be shipped in. The last thing I want to do is replace the valve, it looks like a major
operation to get the valve body out of the machine. As a matter of fact, i would rebuild both lift cylinders for fun before I would
try to remove the valve body.
I could not determine if either cylinder was getting hot at the plunger as if there was fluid bypassing the seal but i plan to
check again.
do you think that the check valve could be a problem?
thanks again.
J

 

[jeff-pa]

thanks, i will check out the thread.

 

[jeff-pa]

thanks for the reply.