How to Diagnose Air Brake Issues in Peterbilt 567

How to Diagnose Air Brake Issues in Peterbilt 567

If you're hauling freight across upstate New York in a Peterbilt 567, the last thing you want is for your air brakes to act up. That “psshh” sound might sound normal, until your stopping power doesn’t feel quite right. Unlike passenger cars, these systems are powered by compressed air, not hydraulic fluid.

That means there’s more that can go wrong and more ways it can fail. And when it does, you're not just risking downtime. You're risking everything. Let’s walk through how to diagnose air brake issues in this heavy-duty beast before they leave you stranded.

Understanding the Air Brake System in the Peterbilt 567

The Peterbilt 567 is built with a robust compressed air brake system that includes multiple components working in harmony. This system consists of an air compressor, air dryer, storage tanks, airlines, service chambers, pushrods, slack adjusters, and S-cams. When functioning correctly, pressing the brake pedal sends compressed air to the service chamber, pushing the diaphragm and activating the S-cam, which expands the brake shoes against the brake drum to create friction.

Each piece has a critical role:

• Air Compressor: Creates the system’s air pressure.
• Air Dryer: Filters moisture to prevent freezing.
• Slack Adjusters: Adjust tension automatically or manually.
• Brake Chambers: Convert air pressure into mechanical force.

Common Symptoms of Air Brake Problems

Spotting a brake issue early is the name of the game. Fortunately, your truck won’t keep quiet about it:

• Hissing or leaking air sounds: Often indicates a broken air line or fitting.
• Slow brake response: Could be a failing air compressor or waterlogged lines.
• Soft or spongy pedal: Might mean excess slack in the system or low pressure.
• Brake drag or uneven braking: Usually from out-of-adjustment slack adjusters or seized S-cams.
• Excessive brake pedal travel: Often the result of worn-out brake shoes or improperly adjusted pushrods.

Step-by-Step: Diagnosing Peterbilt 567 Air Brake Failures

If you're ready to crawl under your truck, here's a field-tested breakdown to diagnose like a pro:

1. Build Full Air Pressure: Let the truck idle until the air tanks reach 120-130 PSI.
2. Listen for Leaks: With the engine off and brakes released, listen near fittings, valves, and tanks for leaks.
3. Apply Brake and Time Pressure Drop: A drop of more than 2 PSI per minute (with brakes released) or 3 PSI (with applied pressure) means a leak.
4. Check Pushrod Stroke: Use a ruler or brake stroke gauge. Excessive stroke means the brakes need adjustment or have a failing component.
5. Inspect Slack Adjusters: Manual slack adjusters should move less than 1 inch. Auto slack adjusters that move excessively may be worn or stuck.
6. Look for Corrosion or Moisture: Check the air dryer’s purge valve and drain tanks regularly, especially during Rochester winters.

How Slack Adjusters and S-Cams Affect Brake Safety

Slack adjusters are mechanical arms connecting the pushrod to the S-cam. Their job? Translate air pressure into mechanical motion. If they’re misaligned or worn out, your brakes won’t fully engage, no matter how hard you stomp the pedal.

S-cams, on the other hand, rotate to spread the brake shoes inside the drum. These cams are shaped like an “S” (hence the name), and they only work effectively when properly greased, centered, and paired with correctly adjusted slack adjusters.

If either component fails, your brakes could lag, drag, or fail outright. And considering the 567’s GVWR, that’s not a risk you want to take.

Quick Checklist for Diagnosing Air Brake Issues

• Build pressure to 125 PSI and shut off the engine
• Listen for audible air leaks near the compressor, valves, and chambers
• Measure pushrod travel—shouldn’t exceed OEM spec (usually ¾”–1½”)
• Check the air dryer function and tank moisture levels
• Inspect the slack adjuster angle and free play
• Test the service and parking brakes individually

Keeping Your Peterbilt 567 Brakes Road-Ready

Diagnosing air brake issues in a Peterbilt 567 requires a methodical approach, starting with a thorough inspection of the valves and chambers, followed by checking for leaks, worn components, or faulty sensors. With Parfitt Tire Service, by combining regular maintenance with prompt troubleshooting, we offer safer operation, reduce downtime, and extend the life of your braking system. If you’re experiencing brake problems or want a professional inspection, contact Parfitt Tire Service today for expert, reliable service that keeps your truck road-ready. Or for more information, read our heavy-duty truck brake guide.