Brake Clearance, Magnets, and Linings
- Set equal brake clearance with slight lining drag.
- Identify magnet wear and documented replacement conditions.
- Inspect linings and apply paired replacement requirements.
- Floor jack and jack stands — Lift and secure the trailer at appropriate support locations, leaving the wheel free to rotate; never use the axle or suspension to support the trailer.
- Wheel chocks — Help prevent trailer movement while lifting and inspecting.
- Brake adjusting spoon — Turn the starwheel through the backing-plate adjustment opening.
- Screwdrivers — Remove the adjustment cover and move the automatic adjusting lever aside when reversing a self-adjusting brake.
- Straightedge — Check magnet faces for uneven wear.
- Caliper — Measure remaining lining thickness for comparison with the documented replacement threshold.
The brake adjustment section calls for adjustment after the first 200 miles and at 3,000-mile intervals afterward, or as use and performance require. Support the trailer correctly on jack stands so the wheel and drum can rotate freely; the axle or suspension must not support the trailer.
Remove the backing-plate adjustment cover and expand the shoes with the starwheel until the wheel becomes difficult to turn. Reverse the starwheel until the wheel turns with slight lining drag; the manual also describes approximately ten reverse clicks. On self-adjusting brakes, move the automatic adjusting lever away with a second screwdriver to permit reverse rotation. Refit the cover and give all brakes the same clearance.
Check magnet faces with a straightedge for uneven wear. Replace a magnet if its coil is exposed or damaged, and replace magnets on both sides of the axle together. The manual specifies genuine Apex replacement parts and calls for refinishing the drum armature when magnets are replaced.
Inspect every shoe and lining for oil, grease, scoring, pitting, or gouging. The manual calls for paired shoe replacement rather than replacing only a contaminated or damaged shoe. Replace a shoe when its lining is thinner than 1/16 inch; heat cracks are described as normally not requiring attention. Lubricant belongs only on the specified moving contact points, not on the friction surfaces.
How It Works
Clearance adjustment and service timing
Adjust after the first 200 miles and at 3,000-mile intervals afterward, or as use and performance require. With the trailer correctly supported and the wheel free to rotate, expand the shoes until rotation becomes difficult, then reverse the starwheel until there is slight lining drag; the manual also describes approximately ten reverse clicks. On self-adjusting brakes, hold the automatic lever away with a second screwdriver during reversal, then refit the cover and give all brakes the same clearance.
Magnet contact and drum condition
The magnet works against the drum armature to actuate the brake, so uneven contact can affect braking action and concentrate wear. Check the magnet face with a straightedge, and replace a magnet if its coil is exposed or damaged. Replace magnets on both sides of the axle together, use genuine Apex replacement parts, and have the drum armature refinished when magnets are replaced.
Lining condition and friction
Inspect every shoe and lining for oil, grease, scoring, pitting, and gouging; contamination changes friction, while surface damage can interfere with contact. Replace a shoe when its lining is thinner than 1/16 inch, and apply the manual's paired shoe replacement requirement rather than replacing only a contaminated or damaged shoe. Heat cracks normally do not require attention, and lubricant belongs only on specified moving contact points because oil or grease on friction surfaces can impair braking.
Step-by-Step Procedure
0/7 doneBrake Adjustment and Wear References
| First brake adjustment | After 200 miles |
| Subsequent adjustment interval | Every 3,000 miles, or as use and performance require |
| Lining replacement threshold | Less than 1/16 inch |
Diagnostic Chart
| Symptom | Check | Spec / Expected | Likely cause & fix |
|---|---|---|---|
| Wheel remains difficult to turn after adjustment. | Verify correct trailer support and free wheel rotation, then reverse the starwheel; move the automatic adjusting lever away when applicable. | Finish with slight lining drag; the manual also describes approximately ten reverse clicks. Refit the cover and give all brakes the same clearance. | Shoes may remain overexpanded, or the automatic adjusting lever may prevent reverse adjustment. Basis: Bostail 8K Axle Manual |
| Magnet face appears uneven, or the coil is visible or damaged. | Place a straightedge across the magnet face and inspect for uneven wear, coil exposure, and coil damage. | Replace a magnet if its coil is exposed or damaged. Replace magnets on both sides of the axle together, use genuine Apex replacement parts, and refinish the drum armature. | Uneven contact can concentrate wear; exposed or damaged coils meet the documented replacement condition. Basis: Bostail 8K Axle Manual |
| A lining appears thin, contaminated, scored, pitted, or gouged. | Inspect every shoe and lining and measure remaining lining thickness. | Replace a shoe when its lining is thinner than 1/16 inch. Use paired shoe replacement rather than replacing only the contaminated or damaged shoe. | Wear reduces lining thickness, while contamination and surface damage can compromise friction and contact. Basis: Bostail 8K Axle Manual |
| Heat cracks are visible in a lining. | Inspect the full lining for contamination, scoring, pitting, gouging, and insufficient thickness rather than deciding from cracking alone. | Heat cracks normally do not require attention; lining thinner than 1/16 inch requires shoe replacement. | Heating and cooling can produce cracks; other documented defects must still be evaluated. Basis: Bostail 8K Axle Manual |
At the Bay: Diagnose the Case
CASE 1 / 4
Wheel remains difficult to turn after adjustment.
- Test
- Compare
- Correct
Select the next check
Basis: Bostail 8K Axle Manual. A likely cause is a hypothesis; confirm it with the applicable service procedure before repair.
Common Bay Mistakes
- ✕ Supporting the trailer by the axle or suspension.
- ✕ Treating approximately ten reverse clicks as a substitute for verifying slight lining drag.
- ✕ Replacing only the defective magnet or shoe instead of applying paired replacement requirements.
- ✕ Allowing grease or oil onto friction surfaces.
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
Dial Indicator — Bearing End Play
Magnetic-base dial indicator (0.001 in) on the hub face, spindle nut adjustment
🛠 Equip your bay for this test →Symptom: Bearing service — set end play after repack.
Spec limits used
- 0.001–0.010 in — Allowable end play, double-nut wheel end (Dexter 9-15K service manual, p.64)
- 0.008 in — Nev-R-Lube 50 mm max axial end play (Dexter 059-831-00)
- illustrative only — Face runout limit (General shop practice)
Simulated readings for practice. Always follow the manufacturer's procedure and values on a real trailer.
Source: Bostail 8K Axle Manual — 36-43