On the way to the facility, the right rear tires caught fire and the tractor and trailer were damaged. The driver had disconnected the brake chamber linkage, assuming that the S-cam would return to the no-brake position (Figure 1A). In fact, the brake was locked and continued driving, eventually caused frictional heating and a fire.

Figure 3 is a view of the brake system from the wheel, indicated by the red arrow in Figure 2 that caused the fire. Thermal patterns indicated this to be the brake drum that had a problem.

Upon removal of the brake drum, one of the brake shoe rollers fell out. The blue arrow in Figure 3 shows the position of the missing roller. The red arrow in Figure 3 shows the S-cam to be in a position nearly overriding the brake shoe roller.

With the S-cam in this position, it has exceeded the travel expected in the design. In this position, the S-cam will not return to the released position (Figure 4), causing uncontrollable brake pressure, severe frictional heating and a fire.

Figure 5 shows severe thermal damage to the brake lining as a result of the jammed S-cam. The root cause of the failure involves worn out brake linings and drum. The linings and the drum had insufficient thickness to prevent the S-cam from traveling into the jammed position.