Should automatic slack adjusters be manually adjusted on trailers or tractors?

Short answer:
➡️ Automatic slack adjusters should NOT be manually adjusted during normal service.

They are designed to self-adjust during proper brake applications. If one needs frequent manual adjustment, there is usually a mechanical problem elsewhere.

✔️ Correct Way to Check Automatic Slack Adjusters

Build air pressure to 90–120 PSI

Apply brakes fully

Measure pushrod stroke:

Type 30 chamber: max 2.0 inches

Type 24 chamber: max 1.75 inches

If stroke is within limits, the brake is correctly adjusted.

🧪 Testing Self-Adjustment

Release brakes

Make 10–15 firm brake applications

Recheck pushrod stroke

✔ Stroke improves → ASA is working
❌ No change → adjustment system is faulty

🚫 Do NOT Manually Adjust ASAs Because:

It hides worn or seized components

Can cause brake drag and overheating

Can lead to uneven braking

Can result in DOT violations

Manual adjustment is allowed only:

After brake repairs

For diagnosis

To move a vehicle to a repair location (where permitted)

🚨 Common Causes of ASA Failure

Seized S-cam bushings

Worn cam tubes

Incorrect chamber size

Low air pressure

Weak brake applications

Frozen or corroded slack adjuster

🛑 DOT Out-of-Service Conditions

Pushrod stroke over limit

Adjuster won’t maintain adjustment

Uneven braking on same axle

These require repair or replacement, not adjustment.

🧠 Pro Tip

If an automatic slack adjuster needs frequent adjustment, it is not functioning correctly. Fix the root cause — don’t fight the adjuster.