On our Cat 375 (1JM prefix) the operator is complaining of the boom settling down after you let go of the control lever (neutral). With the boom up and the bucket off the ground, you can watch it slowly settle to the ground. Any obvious things to check for? The boom cylinders were both rebuilt/resealed about a year ago. Also replaced one of the load control valves on one the cylinders because it was leaking externally due to a crack in the housing.
Either a seal in the drift reduction valve has failed (I replaced them on both of my 345C machines), or one of the boom relief valves has begun to suck the makeup spring into the relief valve seat. A bit about the drift reduction valve (albeit a similar, not same, machine) here.... http://www.catsch.info/320c-excavator-hydraulic-system-boom-drift-reduction-valve.html
Good day Its not so difficult to check out the cylinders I would suggest take the easy work first to be sure. Kind regards Uffex
This machine has a Boom Drift Reduction Valve (BDRV) and a Stick Drift Reduction Valve (SDRV). On each one you've have a relief as heymccal mentions and one of these dead engine lowering valves... Need to make sure they're wound in all the way so the ball (21) seals off the passage... and make sure they don't look like this...
Also a quick and dirty method for checking the relief AND the dead engine lowering valve is to remove the return (T) hose from the BDRV. Cap off the hose and leave the port open to atmosphere. If anything comes out during the drift event, well it can only come from a leaking relief or lowering vale. Just out of curiosity does the stick have the same drift issue?