This S-cam had been over rotated. It also illustrates the pad rollers and the return springs.

One item that most drivers don’t even consider when doing a visual brake inspection is the operation and condition of the “S” cams. If an S-cam is found to be over-rotated, that is an automatic out-of-service violation, and in some jurisdictions, they will remove your license plates, as this type of repair must be carried out by a licensed mechanic before the vehicle is driven again. Unfortunately, most drivers have no idea what the S-cams do or how to check them. When you apply the brakes, the air is supplied to the brake chamber, which extends the pushrod and rotates the slack adjuster. The slack adjuster rotates the S-cam, which pushes the brake pad rollers, extending the brake pads into the brake drums and slowing your truck down. When you release the brakes, various springs return everything to their normal operating positions. If your brakes are out of adjustment, the S-cam rotates further and will eventually rotate beyond its limits, and will no longer be able to apply the brakes, rendering that set of brakes useless. On the upside, visually checking the condition and operation of the S-cam is very easy. On the downside, you will get dirty, as the S-cam is inside the inner part of the rim and connected to the brake pads. The brake pad rollers should be at the narrowest part of the S-cam when the brakes are fully released. If they’re not, then a brake adjustment or repair is needed. If the brake pad rollers are on the inside of the S-cam, do not drive on any public roadway until the issue is corrected, as that set of brakes is completely disabled and will require service from a licensed mechanic, as the automatic slack adjusters will likely also need to be replaced. There is also a good chance the truck or trailer will need a complete brake job, as an inoperative brake will cause extra wear on the operating brakes but no wear on the inoperative brake.

Don Taylor has been a professional driver since March 1985.  In 1994 he made the jump to driving tractor trailers, and has accumulated over 3.5 million miles, including over 4 years of driving turnpike doubles in Manitoba, Saskatchewan and Alberta.  He is currently hauling flat decks across North America.