426 Backhoe Hydraulic Oil Change & Purge Guide

[MikePhua]

Background & Why Purging Matters
On a Caterpillar 426 backhoe loader, changing the hydraulic oil isn’t as simple as draining the tank and refilling. Because the machine has multiple hydraulic cylinders, a significant volume of old oil remains trapped in those cylinders even after draining. As one owner put it: “there is a LOT of fluid in each of those cylinders.”
Leaving the old fluid in the lines and cylinders can defeat the purpose of an oil change, especially if you're concerned about contamination or degraded oil.

What Experienced Users Recommend

1. Filter First, then Drain
   • Change the hydraulic filter before or during the purge process. Quarterly maintenance guidelines for the 426 suggest replacing the filter, cleaning the breather, and maintaining the sight‑gauge level between the “MIN” and “MAX” marks.
     
   • Using a clean filter helps ensure new oil circulates without picking up trapped dirt or sludge.
     
2. Gravity-Drain the Cylinders
   • One practical, lower-effort method is to loosen the hydraulic hoses at the cylinders, then let the old oil flow out by gravity.
     
   • Once drained, refill the tank and gently operate the cylinders (at idle) to purge remaining fluid and air. Work them slowly so you don’t “diesel” (cavitate) the seals.
     
3. Full Line Flush (For Contaminated Systems)
   • If the system has been contaminated by water, metal, or very degraded oil, a more aggressive purge is needed. Some users agree with advice to not just drain cylinders, but also flush all return lines.
     
   • A documented procedure (for a different but similar machine) involves:
     • Draining the reservoir, then partially disconnecting return hoses so they drain into a waste container.
       
     • Running the engine at low idle to pump out about 7 gallons (or the volume of the return and pressure lines).
       
     • Re-connecting hoses, running again to circulate, and finally working cylinders to purge remaining old fluid.
       
4. Watch for Air During Refilling
   • After refilling and running the engine, you need to “burp” or bleed air from the system. Old fluid drains and introducing air can trap air in cylinders. One technique: work the cylinders slowly in both directions at idle, in small strokes, to flush air without letting the cylinder reach full stroke.
     
   • Be cautious: if air is introduced and the purging is not done right, you risk damaging seals.
     

Hydraulic Oil & Quality Considerations

• Use the correct hydraulic oil: Cat’s system requires a fluid with additives like anti-foam, anti-oxidation, and high zinc.
  
• When refilling, use warm hydraulic oil if possible — this helps it flow better and mix with any existing trapped fluid.
  
• Filter all new oil when pouring it in (from barrels or bulk supply) to avoid introducing contamination.
  

Maintenance Intervals & Tips

• Quarterly: replace hydraulic filter, inspect the tank breather, and check the oil level at the sight gauge.
  
• Yearly: change the hydraulic fluid, clean reservoir, and inspect the tank breather.
  
• During any “system invasion” (when you open up filters, lines, or components): use a high-efficiency filter after refilling, and monitor for dirt or air in the system.
  
• Always check for bubbles in the sight gauge after refilling. If bubbles are present, inspect suction hoses and clamps for air leaks.
  

Safety & Practical Advice

• Work on level ground and make sure the backhoe and loader arms are stabilized or locked.
  
• Use proper containment when draining old oil — hydraulic fluid is hazardous waste.
  
• Warm up the engine slightly so the fluid flows, but don’t run it at high RPM during purging.
  
• After bleeding and purging, re-check for leaks and verify the fluid stays within the correct level range.
  

Conclusion
Purging hydraulic oil on a Cat 426 is more involved than just changing the fluid. Cylinders and return lines trap a lot of old fluid, so to do a thorough job, you may need to drain, loosen hoses, and carefully bleed out air after refilling. If the current oil is simply old (not contaminated), a partial purge (drain + gravity bleed) might be sufficient. For contaminated systems, a flush method is preferred. Use quality hydraulic fluid and filters, and always bleed air properly to protect the system.