have had a 966f series 1 with an intermittent boom drop problem. so operator goes into pile backs out with bucket full, if you hold up on joystick boom will lift, when you let go it returns to center but boom will drop at about a foot/second! never has it happened when a mechanic was around to do proper diagnostics. will happen cold or hot. main valve above front axle completely rebuilt with new o-rings and every replaceable valve, check valve, and whatever could be replaced replaced with new. the only thing left is the pilot valve but i don't know how this could be a problem. along with the above problem while pushing into pile some operators say that the bucket will roll back by itself but does not roll out....help...theironoracle
Anyone checked cylinders
Did the rebuild of the control valve include the Ride Control section? The bucket roll could be from not having the floor down flat. That will collaps the cyl against a line relief. Later Bob
are the pilot pressures within spec? may be too high causing the valve not to stay in neutral, cylinders may be bypassing (are the barrels extremly hot after minimal operation?) breakout relief valve may not be adjusted correctly? (set too low - should be higher than main relief) you really need someone to troubleshoot, could be a number of things causing the fault, see it happen alot of times after a valve and/or pump gets a rebuild and the hydraulics don't get checked/adjusted after the pressures are way out and the machine starts playing games.. had a 633D scraper which would turn the steering bit by bit while sitting there at idle to the point of going almost full lock (pilot oil pressure was x5 times too high after a pump overhaul)
has the emergency boom lowering valve been checked, saw the same thing happen on a 992F, thatll do it, check the schematics and then locate it on the machine and check it out.
ps: the valve is usually down low where a service person can access it from the ground, and lower the boom down manually of the machine dies with boom up and cannot be restarted (ie engine/pump failure situation)