No Slacking Off

| November 19, 2004

Brakes need regular attention to ensure they’ll work properly.

More and more equipment on modern big rigs works automatically, and you can drive day after day, week after week, without worrying about it. But don’t let that change the way you think about your brakes. Left alone, they may desert you when you need them most.

Important skills every driver should learn are how to adjust manual slacks, and maintain and inspect automatic slack adjusters.

Nearly all trucks today use cam brakes, which are applied through a camshaft that rotates when you hit the brake pedal. The slack adjuster is a lever that connects the inner end of the camshaft to the diaphragm through a pushrod. The diaphragm itself is located inside the familiar round brake chamber that contains the necessary air pressure. The chamber also contains the springs used to retract the brakes when you release the pedal and to apply the parking brake.

The slack adjuster’s larger end slides onto the camshaft and is connected to it by splines. It’s fastened in place with a large, circular locking clip. This way, it can be slid on and off for replacement, yet the camshaft will be forced to rotate (by the splines) when the slack adjuster is rotated by the pushrod. The slack’s smaller end is connected to the end of the pushrod through a clevis pin that allows the angle between the slack and pushrod to change slightly as the brakes are applied.

The pushrod moves about 11/2 inches to 21/4 inches with a 1/4-inch tolerance as you apply the brakes.

The chamber contains a powerful return spring that is compressed as you apply the brakes. When you take your foot off the brake pedal this spring forces the pushrod back into the chamber and pulls the linings away from the drum so your brakes won’t drag.

As the linings wear even a tiny bit, the stroke of the brake chamber increases a great deal because of the leverage built into the system. A longer-than-normal stroke of the pushrod compresses the return spring more than normal, and the spring then interferes with the brake application force. This will greatly increase your stopping distance, especially if your brakes are overheated.

As soon as the stroke gets too long, the slack lever must be adjusted. Manual slacks are easily adjusted by the driver when he unlocks and rotates a worm gear or screw that rotates the slack lever itself on the splined collar that connects it to the end of the camshaft. Automatic slacks have a mechanism that senses how far the slack moves when the brakes are applied and rotates the adjusting screw automatically with a ratcheting mechanism.

The power of grease
Automatic slacks adjust themselves when they’re working, but they don’t maintain themselves. They need to be looked at – especially to have the stroke checked – almost as often as you used to adjust your old manual slacks. Other than inspection, the biggest part of maintenance is greasing.

Alan LeFever of Pennsylvania Truck Centers in Lancaster, Pa., certified as a master technician by Mack and ASE, showed us the steps necessary to grease and inspect slack adjusters.

The first step is to get the right grease because standard chassis grease may not be suitable. Slacks are often shipped with a special moisture-resistant grease, LeFever says. The manufacturers strongly advise against mixing different lubes unless they meet their exact specifications.

Greasing a slack is simple. Locate and then carefully wipe down the grease fitting (“zerk”). Install the end of your grease gun onto the zerk until it clicks or you can feel it locking. Then slowly pump the handle until grease emerges from somewhere on the slack.

You want to grease until the mechanism is full and grease emerges, meaning the slack is “purged” of air, says Pete Rutt, the service manager at Pennsylvania Truck Centers.

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