I have a cat D6 9U that has one cylinder that isn’t holding. When I lift the blade to the top it levels out but when I release the hand lever to neutral the left cylinder drops rapidly until the right side is holding it up. Obviously the blade is 8-10” out of level which makes it impossible to do any work without it digging too deep on the left side. We have rebuilt both cylinders. The only thing I can think of is if the control valve has separate spool valves and ports for each cylinder and one of the spool valves is bypassing. The end of stroke check valves in the pistons seem to be good. Any ideas? Does anyone know the anatomy of the control valve?
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