How do I properly adjust trailer brakes (manual slack adjusters) on a semi?

I’m looking for the correct, safe way to adjust trailer brakes with manual slack adjusters (not automatic).

My setup:

Tractor + 53’ dry van (drum brakes)

Manual slack adjusters (as far as I can tell)

What I want to know:

What’s the correct step-by-step method to adjust them?

Should I adjust with the wheels off the ground or can it be done on the ground?

What “feel” am I looking for (tight then back off how much)?

How do I check pushrod travel / stroke after adjusting?

Any signs I should stop and replace parts instead of adjusting (cam bushings, worn S-cam, shoes, drums)?

Safety note: I will chock wheels and use jack stands if lifting.

If anyone can share best practice and DOT-friendly checks, I’d appreciate it.

Published Jan 27, 2026 • Ref: #3

Community Updates

Safety First

Park on level ground

Chock wheels front and rear

Release parking brakes (air must be built up)

If lifting, use jack stands — never rely on air suspension

🔧 Manual Slack Adjuster Adjustment Steps
1️⃣ Locate the Slack Adjuster

Found between the pushrod and S-cam

Identify the adjusting bolt (usually hex-shaped)

2️⃣ Tighten Until Brake Shoes Contact Drum

Turn the adjuster clockwise

Rotate until the brake shoes firmly contact the drum

Wheel should be difficult or impossible to turn by hand

3️⃣ Back Off the Adjuster

Back off ¼ to ½ turn
(usually 3–5 clicks depending on design)

Wheel should turn freely with slight drag

4️⃣ Repeat on All Brake Chambers

Adjust every wheel position

Never adjust only one brake on an axle

Jan 27, 2026

Tools Needed

Wheel chocks

9/16" or 7/16" wrench (most slack adjusters)

Brake adjustment tool or flat screwdriver

Tape measure or ruler

Safety gloves / eye protection

Jan 27, 2026

Automatic Slack Adjusters (ASA) — What You Need to Know

Short version:
👉 You normally do NOT manually adjust automatic slack adjusters.
They self-adjust during normal brake applications when everything is working correctly.

Jan 27, 2026

How to CHECK Automatic Slack Adjusters (Correct Way)
1️⃣ Build Air Pressure

Engine running

Air pressure 90–120 PSI

2️⃣ Measure Pushrod Stroke

With brakes fully applied (use a brake buddy or have someone hold the pedal):

Type 30 chamber:

Max allowed stroke: 2 inches

Type 24 chamber:

Max allowed stroke: 1¾ inches

📏 Measure from the brake chamber face to the pushrod clevis.

If stroke is within limits, brakes are in adjustment.

3️⃣ Verify Adjuster Is Working

Release brakes

Apply brakes 10–15 firm full applications

Recheck stroke

✔ Stroke improves = ASA is working
❌ Stroke does NOT improve = problem exists

Jan 27, 2026

How do I properly adjust trailer brakes (manual slack adjusters) on a semi?

Community Updates

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