J&M 275ST - Manual (Page 24)

21
2.0 Service
Replacing Brake Linings:
1. Remove the brake shoe retract spring.
2. Remove the shoe hold down assembly by holding the back of the pin with one hand and pushing against
the spring and twisting with a hold down spring tool until the cup is released.
3. Remove both shoes together leaving the adjuster assembly and spring intact.
4. Clean the backing plate and lever arm.
5. Inspect magnet arm for any loose or worn parts.
6. Replace springs that are broken, bent, or weak.
7. Apply a light film of lubricant to the anchor pin and shoe rest pads & backing plate areas that are in
contact with the lever arm.
8. Attach the adjuster screw and spring to the new brake shoes. The star wheel and adjuster must be
positioned as before.
9. Install the new shoes on the backing plate and reinstall shoe retract spring.
After replacement of brake shoes and linings, the brake must be re-burnished to seat in the new components. This
should be done by applying the brakes 20 to 30 times from an initial speed of 40 m/h, slowing the vehicle to 20
m/h. Allow ample time for brakes to cool between applications. This procedure allows the brake shoes to seat
into the drum surface.
Brake Lubrication:
Before reassembling, apply a light film of lubrication or similar grease, or anti-seize compound on the brake
anchor pin, the actuating arm bushing and pin, and the areas of the backing plate that are in contact with the brake
shoes and magnet lever arm. Apply a light film of grease on the actuating block mounted on the actuating arm.
Troubleshooting:
Most electric brake malfunctions that cannot be corrected by either brake adjustments or synchronization adjust-
ments can generally be traced to electrical system failure. Mechanical causes are ordinarily obvious, bent or bro-
ken parts, worn out linings or magnets, seized lever arms or shoes, scored drums, loose parts, etc. Voltmeter and
ammeter are essential tools for proper troubleshooting of electric brakes.
How to Measure Voltage:
System voltage is measured at the magnets. Connect the voltmeter to the two magnet lead wires at any brake. This
may be accomplished by using a pin probe inserted through the insulation of the wires dropping down from the
chassis or by cutting the wires. The engine of the towing vehicle should be running when checking the voltage (so
that a low battery will not affect the readings).
Brake Magnet Inspection:
Your electric brakes are equipped with high quality electromagnets that are designed to provide the proper force
and friction. Your magnets should be inspected and replaced if worn unevenly or abnormally (Figure 2.7). Even
if wear is normal as indicated by your straightedge, the magnets should be replaced if any part of the magnet coil
has become visible through the friction material facing of magnet. It is also recommended that the drum armature
surface be re-faced when replacing magnets. Magnets should also be replaced in pairs - both sides of an axle.