komatsu d21a-5 hydraulic problem

[jam-pa]

Hello, My machine suddenly has a problem in that the blade will no longer stay in the air, drops to the ground in a few seconds while the machine is running. The hydraulics will hold the machine off the ground when then blade is pushed down so the problem is only in one direction.
i suspect that the problem is in the control valve because i can hear the valve bypassing fluid in the center position. I rebuilt the lift spool with new orings, u-seals and dust seal but the result was no change at all.
does anyone have an idea where to look to solve the problem?
thanks

 

[Dave Neubert]

check the relief valve

 

[jam-pa]

will do.
thanks for the reply.

 

[Dave Neubert]

Also it could be the nut came off the piston on the lift cylinders

 

[jam-pa]

Thanks, I am trying to see if I can isolate the problem to one of the cylinders.