Field case study · West Valley City, UT
Cylinder repack on excavator — West Valley City, UT
Boom cylinder drift and external seal leak on a CAT 320 at a West Valley development site.
Reported symptom
Boom drifted down 3 feet over 10 minutes with the machine idling. External hydraulic oil leaking from the cylinder rod seal. Visible scoring on the chrome rod surface.
Diagnostic finding
Worn piston seals and rod wiper seal caused internal bypass and external leakage. Minor chrome damage from contamination in the hydraulic system (silica from nearby excavation).
Repair scope
Removed and disassembled the boom cylinder in the field. Polished the rod chrome with fine emery cloth to remove minor scoring. Replaced all piston seals, rod seals, wiper seal, and wear bands. Reassembled, pressure-tested, and reinstalled.
Reported result
Boom drift reduced to under 6 inches per hour (within spec). No external leaks. Machine returned to digging footings same day. Hydraulic oil sample recommended to check for ongoing contamination.
What this case helps clarify
This job is a useful example of why the first reported symptom is only the starting point. The machine type, brand, location, operating conditions, and observed diagnostic finding all shape the repair path. A similar complaint on another model may require a different test sequence or a different field-versus-shop decision.
For a new service request, send the machine make and model, hours if known, warning codes or dashboard photos, the conditions that trigger the problem, and the safest access point at the jobsite. That information helps the technician prepare the right tools and determine whether the work can be performed where the machine sits.
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