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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.

Slacks should be greased as frequently as the rest of your chassis – at least as often as you change your oil and check coolant SCAs, or perhaps twice as often if running extended oil change intervals.

Adjusting manual slacks
Many older trucks and trailers still have manual slacks. Adjustment needs to be done frequently – at least once a week in hard service.

It’s essential to adjust brakes when they are cold as brakes tighten up as they cool off. Brakes adjusted to the right stroke when hot may drag when they cool.

The proper stroke should be listed in your owner’s manual, according to the size of the brake chamber. The size of the chamber is marked on the outside. Long stroke chambers are marked with similar numbers. You can also look up stroke measurements in the Commercial Vehicle Safety Alliance inspection criteria.

Manual slacks have an adjusting bolt and locking ring on the side. First, thoroughly wipe down all the slacks so you can work on them. To adjust, first get a box wrench the right size for the adjusting bolt. A box wrench works best as the shape is ideal for depressing the adjusting bolt’s locking ring.

Next, securely chock the truck’s wheels with chocks in both directions so it won’t roll. Idle the engine to build up air pressure and then depress the parking brake buttons so the brakes will be released.

Once the truck is secured and brakes released, you can adjust each slack.

  1. The first step is to mark the pushrod so you can observe the stroke. Mark it a convenient distance away from the side of the chamber.

  2. Next, install the box wrench onto the adjusting bolt and with your thumb force it downward far enough to release the locking ring.
  3. Turn the wrench while watching the pushrod. Turn it in the direction that causes the pushrod to come out of the brake chamber. Rotate the adjustment until you feel strong resistance.
  4. Normally, you’ll need to back the adjusting bolt off 1/4 to 3/8 turn. You may want to first remove and reinstall it at a 90-degree angle to the slack so you can better estimate how far you’ve turned it. Then turn it 90 degrees in the opposite direction used to tighten the brakes. Rotate it slightly more if necessary, until the locking ring pops out.
  5. Now, check the stroke. The ideal is to have someone apply the brake pedal for you with 90 psi in the system. But if you can’t manage that, you can pry the pushrod out of the chamber hard with a long prybar. If adjusting with 90 psi to apply pressure, use the standard maximum (minus the 1/4-inch allowance for inspection purposes). But if using a prybar, since you can’t apply as much brake force manually, look for a stroke slightly shorter than the standard adjustment. If the stroke is about 1/4 inch below the standard allowance but there is still obvious stroke, your adjustment will be safe.
  6. To actually measure stroke, place the end of a ruler or tape measure against the face of the brake chamber. Note or mark the position of your pushrod mark on the scale. Then apply the brakes as described above. Note the new measurement. Subtract the larger dimension from the smaller one to get the stroke.

Checking auto slacks
First identify the clevis. This is the U-shaped fitting that is fastened onto the end of the brake chamber’s pushrod by threads and a locknut. The clevis pin connects the slack adjuster’s smaller end to the pushrod and allows the pushrod to rotate in relation to the slack when the brakes are applied, as mentioned above. On ArvinMeritor slacks, there is a smaller pin at the outer end of the clevis. This connects a link to the clevis. This link connects to the adjustment mechanism inside the slack at the other end.

Inspect both pins and wiggle the clevis (and on ArvinMeritor slacks the link) up and down if you can. There should be very little play. If play is excessive, the brakes may not adjust properly until the pin or clevis is replaced. Also, grasp the big end of the slack and attempt to rock the end of the camshaft up and down. There should be very little play here, as well. If there is play, the camshaft bushings need replacement.

Next, check installation angle. Generally, the pushrod should be at 90 degrees to the slack when the brakes are released. This must be done with two of the wheels securely chocked and the parking brakes released.

On Haldex slacks, if installation and adjustment are correct, the pointer on the end of the camshaft will be aligned with the notch in the side of the slack.

On ArvinMeritor slacks, this check is made with a cardboard template. The lettering will be visible when checking the left side slacks. When checking those on the right side, flip the template over so you are looking at the plain side. You align the correct hole in the narrow end of the template (depending on slack length) with the hole in the camshaft. You can use a center punch to hold it in place. Then align the large, round hole in the bigger end of the template with the clevis pin. Finally, look for the smaller pin in the outer end of the clevis through the small, oval hole in the template. If the smaller pin is somewhere inside the oval hole, the installation angle and function pass the test.

One thing to check if alignment is not correct is that the parking brake retracts the pushrod fully. If not, you’ll need a new brake chamber.

The next step is to mark the slack and check brake stroke as described above. If the stroke is all right, the slack is OK.

What do you do if the stroke is excessive? Unfortunately, looseness in not only the camshaft but also in other brake parts inside the brake drum, or other problems like defective return springs there, can cause the stroke to be too long.

Follow this procedure to check slack function:

  1. On Haldex (and many other slacks), install the right size box wrench on the adjusting bolt. Rotate the bolt in the direction that will loosen the adjustment. If the slack is OK, there will be an obvious torque – 13-15 pounds feet, as well as a very obvious ratcheting. If the adjustment turns easily without the ratcheting, the mechanism is worn, and the slack should be replaced.

  2. On ArvinMeritor slacks, gently attempt to rotate the adjustment in the direction of loosening it (don’t release the pawl as described below). If there is noticeable resistance, the mechanism is probably OK. If not, replace the slack.
  3. Next, check slack function this way: First, you need to back off the slack’s adjustment a full 1/2 turn. On the Haldex, do it just the way you tested the function, by backing off against the ratcheting action. On the ArvinMeritor slack, first pry the pawl out of the slack adjuster housing with a large, flat-bladed screwdriver. Retract the pawl until you feel resistance and hold it all the way out. The pawl is designed to prevent rotation of the mechanism except in the forward direction. If you don’t fully retract the pawl and you forcibly back off the adjustment, you will damage the mechanism. Now, back off the mechanism a 1/2 turn and release the pawl.
  4. Next, get a white marker and paint a line along the top of the adjusting bolt. Now, have somebody repeatedly apply the brakes using 90 psi pressure. Watch the bolt. It should turn steadily in small increments, with the rotation slowing toward the end of the process. After five or six brake applications, the original position of the adjusting bolt should be restored. If the slack adjusts up this way and passes the above torque tests, the most likely explanation for incorrect stroke is worn parts in the rest of the system.

If a slack quits functioning, you can adjust it just as with a manual slack, as described above. You need not perform any special procedure to protect the mechanism when turning the adjustment forward. But LeFever reports that if the slack is very badly worn, it may not hold the adjustment.

For further information
Bendix Heavy Vehicle Systems

(248) 435-1000

(800) 677-3786

Haldex Commercial Vehicle Systems
(816) 891-2470

Crewson Brunner, Inc.
(716) 894-1044

Pennsylvania Truck Centers
(717) 393-3633

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