CAT 938K Parking Brake Pad Replacement and Axle Service Strategy

[MikePhua]

The CAT 938K and Its Mid-Size Loader Legacy
The Caterpillar 938K wheel loader was introduced as part of CAT’s K-series lineup, designed to meet Tier 4 Final emissions standards while improving fuel efficiency, operator comfort, and hydraulic responsiveness. With an operating weight of approximately 35,000 lbs and a net power rating of 188 hp, the 938K is widely used in aggregate yards, municipal fleets, and construction sites. Its powertrain includes a torque converter, automatic transmission, and a wet-disc braking system integrated into the axle housing.
The parking brake on the 938K is a spring-applied, hydraulically released disc system mounted within the rear axle assembly. Over time, the brake pads wear down, leading to reduced holding force, warning alerts, or failure to engage. Replacing these pads requires careful disassembly and attention to hydraulic and mechanical tolerances.
Terminology Notes

• Wet Disc Brake: A brake system where friction discs operate in an oil bath, reducing wear and improving cooling.
  
• Spring-Applied Brake: A fail-safe design where springs engage the brake when hydraulic pressure is lost.
  
• Axle Housing: The structural casing that contains the differential, brake components, and planetary gears.
  
• Carrier Assembly: The internal gear and bearing structure that supports the axle shafts.
  
• Brake Piston: A hydraulic actuator that compresses the brake discs to release the parking brake.
  

Symptoms of Worn Parking Brake Pads
Common signs include:

• Parking brake warning light on dash
  
• Loader rolls slightly when parked on incline
  
• Audible grinding or squeal during engagement
  
• Brake fails to release smoothly
  
• Diagnostic codes related to brake pressure or travel
  

In one municipal fleet in Ohio, a 938K began rolling backward on a slope despite the parking brake being engaged. After inspection, technicians found the brake pads worn below minimum thickness and the piston seals beginning to leak.
Disassembly and Replacement Procedure
To replace the parking brake pads:

• Park loader on level surface and block wheels
  
• Disconnect battery and relieve hydraulic pressure
  
• Drain axle oil and remove rear axle cover
  
• Extract carrier assembly and brake piston
  
• Remove worn pads and inspect discs for scoring
  
• Install new pads with OEM thickness spec
  
• Replace piston seals and reassemble with fresh oil
  
• Test brake engagement and release under load
  

Recommended tools:

• Torque wrench with CAT spec chart
  
• Seal driver and snap ring pliers
  
• Brake pad thickness gauge
  
• Hydraulic pressure tester
  
• Clean oil pan and magnetic drain plug
  

In one quarry operation in Alberta, a 938K was serviced after brake fade was noticed during stockpile loading. The technician replaced the pads and added a magnetic plug to monitor future wear debris.
Pad Selection and Material Considerations
Brake pads for the 938K are typically semi-metallic or sintered composite, designed for wet-disc environments. Selection tips:

• Use OEM or certified aftermarket pads with correct friction rating
  
• Avoid organic pads in high-load applications
  
• Verify pad thickness and backing plate integrity
  
• Replace both sides to maintain balance
  

In one farm operation in Georgia, switching to sintered pads improved brake hold on wet clay slopes and reduced fade during long idle periods.
Preventive Maintenance and Inspection Strategy
To extend brake life:

• Check parking brake function weekly
  
• Inspect axle oil level and condition monthly
  
• Replace axle oil every 1,000 hours or annually
  
• Monitor brake pressure sensor readings
  
• Clean axle breather to prevent pressure buildup
  

Recommended upgrades:

• Install brake wear sensor for early alerts
  
• Use synthetic axle oil for better thermal stability
  
• Add service log for brake inspections and pad changes
  

In one grading crew in Texas, adding a brake wear sensor reduced unplanned downtime and improved safety compliance during DOT inspections.
Final Thoughts
The parking brake system in a CAT 938K is more than a safety feature—it’s a mechanical safeguard that protects operators, equipment, and jobsite integrity. Replacing worn pads requires precision, clean work, and an understanding of hydraulic behavior. In the rhythm of loading and hauling, a reliable brake isn’t just a stop—it’s the confidence to move forward, knowing the machine will hold when it must.