Overview
The GM 14-bolt full-floating rear axle was available in heavy-duty Square Body trucks from 1973-1987, primarily in 3/4-ton and 1-ton applications (C20, K20, C30, K30). While robust and reliable, the factory brake system on these axles often requires modification for improved performance, especially when used with larger tires, heavy loads, or upgraded powertrains. This article covers drum-to-disc conversions, upgraded drum systems, and performance enhancements specific to Square Body 14-bolt applications.
The 14-bolt axle used two primary brake configurations during the Square Body era:
- 11" x 2.75" drum brakes (standard on most applications)
- 12" x 2.75" drum brakes (heavy-duty trucks and some late-model applications)
Modern brake modifications significantly improve stopping power, reduce fade, and enhance overall safety, making them essential upgrades for restored or modified Square Body trucks.
Common Issues
Factory Drum System Problems
- Brake fade under heavy use or with oversized tires
- Poor wet weather performance due to water retention in drums
- Adjustment difficulties with worn or seized adjusters
- Uneven wear patterns causing pull or grabbing
- Limited heat dissipation leading to glazed shoes and drums
Age-Related Issues (1973-1987 Trucks)
- Corroded brake lines and fittings requiring complete replacement
- Seized wheel cylinders from decades of moisture exposure
- Worn backing plates with damaged mounting surfaces
- Contaminated brake shoes from leaking axle seals or wheel cylinders
Parts & Materials Needed
Disc Brake Conversion Kits
Wilwood Forged Dynalite Kit
- Part Number: 140-11388
- Includes: 12.19" rotors, 4-piston calipers, brackets, hardware
- Requires: 15" minimum wheel diameter
- Price Range: $800-1,000
SSBC A126-4 Disc Conversion
- Part Number: A126-4
- Includes: 12" rotors, single-piston sliding calipers
- Compatible with 14" wheels
- Price Range: $600-800
CPP 14-Bolt Disc Kit
- Part Number: 6567DBK
- Includes: 13" rotors, loaded calipers, stainless lines
- Bolt-on installation
- Price Range: $700-900
Drum System Upgrades
Heavy-Duty Drum Kit Components:
- Raybestos 12" Drums: Part Number 9584R
- Wagner Severe Duty Shoes: Part Number Z784
- ACDelco Wheel Cylinders: Part Number 18E1318
- Dorman Hardware Kit: Part Number HW17361
Tools & Materials Required
- Brake line flaring tool (double flare type)
- Brake bleeding kit or vacuum pump
- Torque wrench (1/2" drive, 50-150 ft-lbs range)
- DOT 3 brake fluid (1-2 quarts)
- 3/16" and 1/4" brake line (as needed for routing)
- Thread locker (medium strength)
Step-by-Step Guide
Disc Brake Conversion Process
Phase 1: Preparation and Removal
-
Safety Setup
- Park on level ground and engage parking brake
- Chock front wheels securely
- Raise rear of truck and support with jack stands on frame rails -
Remove Factory Components
- Remove rear wheels and drums
- Disconnect brake lines at backing plate junction
- Remove backing plate bolts (typically 1/2" or 9/16")
- Clean axle housing mounting surface thoroughly
Phase 2: Disc Brake Installation
-
Mount Disc Brake Brackets
- Test-fit brackets to axle housing
- Apply thread locker to mounting bolts
- Torque bracket bolts to 75-85 ft-lbs (verify kit specifications) -
Install Rotors and Calipers
- Slide rotors onto axle shafts (full-floating design)
- Mount calipers with provided hardware
- Torque caliper mounting bolts per kit instructions (typically 35-45 ft-lbs) -
Brake Line Routing
- Route new brake lines away from exhaust and suspension components
- Use factory mounting points where possible
- Install new brake hoses with proper orientation
Phase 3: System Integration
-
Master Cylinder Considerations
- 1973-1980: May require dual-diaphragm booster upgrade
- 1981-1987: Factory master cylinder typically adequate
- Disc conversion reduces line pressure requirements -
Proportioning Valve Adjustment
- Install adjustable proportioning valve if not factory-equipped
- Initial setting: 50% rear bias
- Fine-tune after road testing
Drum System Upgrade Process
Heavy-Duty Drum Installation
-
Drum Preparation
- Have new drums turned to ensure roundness
- Clean with brake cleaner to remove protective coating -
Shoe and Hardware Installation
- Replace all return springs and adjusters
- Apply high-temperature brake grease to contact points
- Pre-adjust shoes to approximately 0.030" clearance -
Wheel Cylinder Service
- Rebuild or replace all wheel cylinders
- Torque mounting bolts to 15-20 ft-lbs
- Bleed individual cylinders before final assembly
Year/Model Differences
1973-1980 Square Body Applications
- C20/K20: Standard 11" drums, single-piston master cylinder
- C30/K30: 12" drums standard, vacuum assist may be manual
- Brake Lines: Steel lines throughout, prone to corrosion
- Proportioning: Fixed proportioning valve in most applications
1981-1987 Square Body Applications
- Improved Master Cylinder: Larger bore, better pedal feel
- Updated Proportioning: Height-sensing proportioning valve on some K20/K30
- Line Protection: Better coating on brake lines
- Hardware Changes: Metric fasteners on some components
Model-Specific Considerations
C20/C30 (2WD Applications)
- Simpler brake line routing
- No transfer case clearance issues
- Often equipped with load-sensing proportioning
K20/K30 (4WD Applications)
- More complex line routing around front differential
- Transfer case may interfere with some conversion kits
- Additional protection needed for brake lines
Suburban Applications
- Higher load requirements demand upgraded systems
- Longer wheelbase affects brake balance
- Often equipped with trailer brake controllers
Tips & Best Practices
Installation Best Practices
- Always replace brake fluid during any brake system work
- Use proper torque specifications - over-tightening can warp rotors
- Break in new components properly - 200 miles of moderate use
- Avoid mixing brake component brands when possible
Performance Optimization
- Upgrade to stainless steel brake lines for longevity and performance
- Consider upgraded brake pads for specific use (towing, performance, etc.)
- Install brake cooling ducts for heavy-duty applications
- Use high-temperature brake fluid (DOT 4) for severe service
Cost-Saving Approaches
- Rebuild existing wheel cylinders instead of replacing (if cores are good)
- Turn existing drums if within service limits (typically 0.060" over)
- Reuse hardware that passes inspection
- Shop multiple vendors - prices vary significantly
Common Mistakes to Avoid
- Mixing DOT 3 and DOT 4 fluids (use one type consistently)
- Ignoring brake line condition during upgrades
- Inadequate bleeding leading to spongy pedal feel
- Incorrect proportioning causing premature lock-up
Troubleshooting
Disc Brake Conversion Issues
Spongy Pedal Feel
- Cause: Air in system or incorrect master cylinder
- Solution: Complete system bleeding, verify master cylinder compatibility
Excessive Pedal Travel
- Cause: Incorrect caliper piston adjustment or worn master cylinder
- Solution: Adjust calipers, rebuild/replace master cylinder
Brake Pull or Grab
- Cause: Uneven pad contact or warped rotors
- Solution: Resurface rotors, verify caliper alignment
Drum System Problems
Poor Stopping Power
- Cause: Glazed shoes/drums, incorrect adjustment, or contamination
- Solution: Resurface drums, replace shoes, check for fluid leaks
Brake Drag
- Cause: Over-adjusted shoes or seized wheel cylinder
- Solution: Readjust shoes, service wheel cylinders
Noise During Braking
- Cause: Worn shoes, contaminated surfaces, or loose hardware
- Solution: Inspect and replace worn components, clean surfaces
System-Wide Issues
Master Cylinder Problems
- Symptoms: Fluid loss, spongy pedal, or poor pressure
- Diagnosis: Check for external leaks, test pedal firmness
- Solution: Rebuild or replace master cylinder, upgrade if undersized
Proportioning Issues
- Symptoms: Rear wheels lock prematurely or insufficient rear braking
- Diagnosis: Road test with gradual braking force increase
- Solution: Adjust proportioning valve, verify correct valve type
Brake Line Failures
- Symptoms: Fluid leaks, corrosion, or damaged lines
- Diagnosis: Visual inspection of entire brake system
- Solution: Replace affected lines, upgrade to stainless steel or NiCopp
This comprehensive approach to 14-bolt brake system modifications ensures Square Body owners can achieve modern braking performance while maintaining the reliability and character of their classic trucks.