I've had this machine for 15 years. The problem was very very slight at first then got worse as the years went by. What it's doing is: The backhoe boom sinks down quickly when in the neutral position. It also sinks down while the boom up function is in use and must be held open while digging or swinging to keep from sinking. The dealer has nobody that has any real experience on anything this old. Geeeze its only 31 years old... So the dealer can only guess at it. Could be anything they say. I've had the boom cylinders rebuilt with no change. The dealership has cheerfully invited me to bring it to them for repair. Yeah Right... They are already guessing and have told me they have nobody working there from back in the day that knows what that set of symptoms would indicate for a part failure. Could be this could be that... will not cost more than one million dollars for them to guess it fixed... The service manual parts breakdown shows top and bottom relief valves with load checks, 2 anti cavitation valves and the spool all in the boom circuit body. It also has a boom lock function that does not work. I have tried adjusting both relief valves to a higher pressure with no change and yes I put them back to the same starting start position. I'm not even sure which relief is supposed to hold the boom up, top or bottom. Lifting the machine with down pressure holds rock steady with no sink down whatsoever. I really don't want to pull the entire valve bank to figure this out. Can anybody give me a solid clue as to which part of the boom system has failed? I guess if it really is the spool them i'm going in deep... Thanks Guys
The spool is not very complicated & you don't have to take the whole bank apart to fix it if that's the problem. I've done a couple but not on your particular model others here I'm sure can give you sound advice.
Today I swapped the top relief valve for the bottom. No change. Boom still sinks right to the ground in a matter of seconds. At my wits end with this thing.
I think the boom only has 1 cylinder is it possible they buggered up with the seal kit on the piston ? Certainly sounds from your description that may be the case. Googled some pics. now I know you have 2 cylinders still even if one of the pistons seals are bad the boom will drop. I'm not sure but maybe if you run it for a while then feel both cylinders if one is warmer than the other it may indicate an internal bypass in that cylinder. Good luck sure hope you get this solved.
Did you find out what the problem was.
Your problem may well be a bad load check valve. If the O rings and back up ring on the load check are missing or broken the boom will fall. See illustration, you will need Items 13, 19, 20, 23. They are the same for the top and bottom load checks. Load checks are behind the relief valves.
Nice to see you posting again gg.