Set Up the Documented Automatic Slack Adjuster
- Select the setup procedure for the identified adjuster.
- Perform the documented temporary-shim and pin-installation sequence.
- Reject and restart an assembly that fails the drum-rotation check.
- Hand wrench — Move the adjuster through the documented setup sequence.
- Socket and ratchet set — Remove and install chamber and adjuster mounting hardware as required.
- Pliers — Install and secure the cotter pins.
- Inspection light — Inspect clevis alignment, pin seating, and cotter-pin security.
- Wheel chocks and rated support stands — Secure and support the trailer before working where the drum must turn freely.
Use the adjuster manufacturer's installation template when one is available. The source supplies an alternative sequence when no template is available, but also notes that automatic-adjuster designs differ.
The alternative starts with a chamber having the proper pushrod length and an adjuster fitted to the camshaft. As the adjuster arm reaches the clevis slot, the pivot pin temporarily acts as a shim while the arm is adjusted against it.
Once the arm contacts the temporary pin, continued wrench movement rotates the camshaft and brings the shoes toward the drum. Rotate the drum by hand to assess drag, then continue to the source's specified stopping and additional-adjustment points.
Dexter's sequence uses a 1/2-inch pivot pin, an additional 1/4–1/2 wrench turn after the drum stops, and a check that the drum can rotate one full revolution after pivot-pin installation. If it cannot, remove the chamber and adjuster and restart with less tightening. A 1/4-inch pin and secured cotter pins finish the sequence. Values vary by maker.
How It Works
Identify the design before choosing the method
Use the adjuster manufacturer's installation template when available; the documented alternative is not a universal automatic-adjuster procedure. Begin that alternative only with the applicable adjuster identified, the chamber having the proper pushrod length, and the adjuster fitted to the camshaft.
Understand the temporary-shim action
As the arm reaches the clevis slot, the pivot pin temporarily acts as a shim against which the arm is adjusted. After contact, continued wrench movement rotates the camshaft and moves the shoes toward the drum, so turning the drum by hand provides feedback on increasing drag. In Dexter's sequence, continue until the drum stops, then apply an additional 1/4–1/2 turn.
Prove the setup before finishing
Pull the adjuster away from the temporary pin, remove the pin, align the pivot holes, and install the pivot pin; Dexter specifies a pivot pin of 1/2 inch diameter. Check that the drum can rotate one full revolution; if it cannot, remove the chamber and adjuster and restart with less tightening rather than forcing acceptance. Finish with the remaining pin of 1/4 inch diameter and install and secure the cotter pins.
Step-by-Step Procedure
0/8 doneDexter automatic-adjuster setup values; values vary by maker
| Pivot pin | 1/2 inch diameter |
| Additional wrench rotation after drum stops | 1/4–1/2 turn |
| Required drum rotation check | One full revolution |
| Remaining pin | 1/4 inch diameter |
Diagnostic Chart
| Symptom | Check | Spec / Expected | Likely cause & fix |
|---|---|---|---|
| The adjuster design or applicable setup method is uncertain. | Identify the adjuster and determine whether its manufacturer's installation template is available. | Use the manufacturer's template if available; otherwise use the documented alternative only for the applicable adjuster. | An installation sequence for a different adjuster design may have been assumed to apply. Basis: 9-15k complete service manual |
| The installer cannot confirm how far adjustment continued after the drum stopped. | Review whether the drum was turned by hand during adjustment and whether the additional wrench movement was controlled. | Dexter additional wrench rotation after drum stops: 1/4–1/2 turn. | The stopping point or subsequent wrench movement was not tracked during setup. Basis: 9-15k complete service manual |
| The drum cannot complete the required rotation after pivot-pin installation. | Attempt the drum-rotation check by hand. If it fails, remove the chamber and adjuster and restart with less tightening. | One full revolution | The setup may have been tightened too much; the source directs restarting with less tightening. Basis: 9-15k complete service manual |
| Pin installation or retention is incomplete. | Verify that the temporary shim was removed before pivot-hole alignment, the pivot pin and remaining pin are installed, and the cotter pins are secured. | Dexter pivot pin: 1/2 inch diameter; remaining pin: 1/4 inch diameter; install and secure cotter pins. | The temporary-shim stage was confused with final pin installation, or final retention was omitted. Basis: 9-15k complete service manual |
At the Bay: Diagnose the Case
CASE 1 / 4
The adjuster design or applicable setup method is uncertain.
- Test
- Compare
- Correct
Select the next check
Basis: 9-15k complete service manual. A likely cause is a hypothesis; confirm it with the applicable service procedure before repair.
Common Bay Mistakes
- ✕ Applying Dexter's alternative sequence to an unidentified adjuster.
- ✕ Starting without confirming the proper chamber pushrod length.
- ✕ Confusing temporary-shim placement with final pivot-pin installation.
- ✕ Accepting a failed drum-rotation check or leaving cotter pins unsecured.
Quick Check
Practice only — not scored.
Plan the Service Sequence
Planning practice only. Follow the applicable manufacturer's instructions, employer safety program and qualified supervision on real equipment.
Plan the first action. Compare the service steps with the bay mistakes.
Hands-on: Shop Test Equipment
Air Brake Board — Leakdown & Pushrod Stroke
Supply and service gauges, gladhand controls, pushrod stroke ruler (Type 30 chamber)
🛠 Equip your bay for this test →Symptom: Pre-trip inspection.
Spec limits used
- 2 in — Type 30 max pushrod stroke (Dexter 9-15K service manual, p.52)
- 80–90 psi — Adjustment test application pressure (Dexter 9-15K service manual, p.51)
- 130 psi — Never exceed (chamber failure risk) (Dexter 9-15K service manual, p.51)
- illustrative only — Leakdown pass limit (General shop practice)
Simulated readings for practice. Always follow the manufacturer's procedure and values on a real trailer.
Source: 9-15k complete service manual — 45, 50