As brake shoes wear down from use, the hydraulic wheel cylinder has to push its pistons out farther in order for the shoes to contact the inner diameter of the drum. When the brake lining of the shoe wears away, metal-to-metal contact will occur and can damage the brake drum. There are a couple of shoe styles--one riveted, the other bonded--and they require slighly different ways of measuring. You can also measure the width of an installed shoe to compare it to interior drum diameter in the installation of drum brakes.
Clean all the rivets on riveted brake shoes prior to measurement. Much of the friction material that wears away can become trapped and pressed inside the rivets. Use a small straightedge screwdriver to cleans the rivets out thoroughly.
Use a tire tread depth gauge. Insert it into each rivet. There can be more than eight rivets on a brake shoe. Measure each by placing the flat base of the tire tread depth gauge onto the brake lining and then pushing down on the sliding ruler. Compare the measurements of each rivet in 1/32 inches to determine even or uneven brake shoe wear.
For bonded shoe measurements, use a brake lining gauge. Bonded shoes do not employ rivets; they are glued to the shoe plates. This style of shoe has to be measured from the edges of the brake shoe lining. Take several measurements along the edge of the shoe to compare even or uneven wear. Since there are a few different brake lining gauges, each may employ slightly different procedures. Some use a scissor motion to pinch the actual lining in the end of the tool and compare the measurement on the opposite, wider end. Other gauges simply have a preset thickness and offer different sizes on the same tool.
Use a Vernier caliper tool to measure the inside diameter of the brake drum being placed onto the rear brake shoes. The caliper tool employs two sides: one for the interior diameter measurement, the other for the exterior diameter measurement. Use the internal measuring side to measure the inside diameter of the drum, then record the measurement.
Manually adjust the opposite exterior diameter side of the caliper and reduce the measurement by 0.024 inches.
Place the exterior ends onto the brake shoe width and manipulate the self-adjuster mechanism on the brakes until the shoes hit each side of the caliper evenly. Be sure to place the caliper ends on the parts of the curved shoes that protrude outward the most. This will allow easy drum installation on the shoes.