Brake It Down

Think about stopping for a moment. You hit the brake pedal countless times a day and it never jumps into your mind that your brakes might not hold. Good thing to keep it that way. Proper brake care and checking can keep that peace of mind.

Brake stroke
One of the most critical items of roadside inspections is the stroke of the brake chambers. Brake stroke is the distance the chamber diaphragms and pushrods move from their released position before the brakes are fully applied.

Why is stroke so critical? Because braking force depends upon the length of that stroke. The longer the stroke, the less the force. An excessive stroke also delays the application of the brake, and it uses more air from the system.

Braking force diminishes as the stroke increases because of the behavior of the return springs that keep the brakes off when you release the treadle valve. (The treadle valve is operated by the brake pedal.)

If the stroke is a little too long when the brakes are cold, it will be far too long when they get hot. Why are there runaway ramps for heavy trucks on long downgrades? Because long grades make the brake drums get hot. For many trucks that end up in runaway ramps, the problem was too much brake stroke.

The ArvinMeritor stroke-sensing slack adjuster adjusts when the brakes are released.

Slack adjusters
Slack adjusters are lever arms that convert the pushing motion of the brake chambers into rotating force on the ends of the camshafts. As the camshafts turn, their cams force the shoes against the drum, via rollers. When brake linings wear, the slack adjusters have the job of rotating the position of the camshaft in relation to the slack’s lever arm to compensate. The camshaft will be turned a little in the direction of application to “take up the slack” in the brakes, and keep the chamber stroke constant.

Not so long ago all drivers had to adjust their brakes at the slack adjusters by turning an adjusting bolt with a wrench. Automatic slack adjusters, that mechanically measure their own stroke and constantly adjust this bolt tighter as the brake linings wear, were mandated by the federal government in 1994.

Daily inspection
Good brake maintenance begins with a daily pretrip inspection. “The normal pretrip seems to be just a quick check of lights and a chance to beat the tires to check for low air pressure. You really need to get under the truck and go much deeper,” Jim Feddern, Ohio State Highway Patrol enforcement manager and chair of the vehicle committee at the Commercial Vehicle Safety Alliance, says.

He recommends the driver check for air lines rubbing the frame or other parts, cracked linings, and cracks or other problems on the inner surfaces of the drums. Also, check the mounting of the brake chambers and the linkage connecting them to the slack adjusters.

“Adjustment of the manual slacks that are still around should be done once a week for over-the-road trucks and as often as every two or three days if you’re running in the mountains,” Feddern says.

Jim Szudy, engineering manager vehicle systems at Bendix Heavy Vehicle Systems, recommends not only a check of brake stroke, but an air leakage check. It’s done two ways.

You first start the engine and build up full brake pressure. Then shut the engine down, and watch the air pressure gauge while timing. Air pressure should not drop more than 6 psi in two minutes for a tractor and single trailer.

Repeat the test after running the pressure back up to the maximum with the engine. (Make sure to chock the wheels so the truck won’t roll.) This time, release the parking brakes allowing time for filling of the parking brake chambers, and then apply the service brakes. This requires using a helper or some sort of device, which holds the treadle valve fully on. Then shut the engine down again. Check again to make sure the pressure drops 6 psi or less in exactly two minutes. If the pressure drops 7 psi or more in those two minutes, there is a leak somewhere. If this only occurs with parking and service brakes applied, the problem is in a hose, hose connection, brake chamber diaphragm, or relay valve.

John Ferry, general manager of Transaxle, Inc., recommends you walk around the truck listening for leaks. Leaks must be fixed to ensure adequate brake system performance.

Checking stroke
The final and most critical brake-related pretrip check is checking brake stroke. Even if you find it’s impractical to check the stroke every day, check it at least once a week.

There are a number of simple stroke check devices on the market, but actual stroke measurement is the most precise and the method used by roadside inspectors. First, measure the distance from the face of the brake chamber on the pushrod side to the center of the pushrod clevis pin at each wheel. The clevis pin is the part that links the pushrod to the top of the slack adjuster.

Then, get someone to apply the brakes at 90-psi. If brake pressure is lower than this, run the engine without a brake application long enough to get up to this pressure. If it’s higher, turn the engine off and apply the brakes a couple of cycles to bring it down to this level. Then, apply the brakes and again measure the distance from the face of the brake chamber to the center of the pushrod clevis pin. Subtract the difference to determine the stroke.

Here are the latest CVSA stroke limits.

Standard, clamp-type chamber adjustment limits:

  • Type 20 (6 25/32-inch diameter) = 1 3/4-inch stroke
  • Type 24 (7 7/32-inch diameter) = 1 3/4-inch stroke
  • Type 30 (8 3/32-inch diameter) = 2-inch stroke
  • Type 36 (9-inch diameter) = 2 1/4-inch stroke

Long-stroke type chamber adjustment limits:

  • Type 20 (6 25/32-inch diameter) = 2-inch stroke
  • Type 24 (7 7/32-inch diameter) with less than 3-inch maximum stroke = 2-inch stroke
  • Type 24 (7 7/32-inch diameter) with 3-inch maximum stroke = 2 1/2-inch stroke
  • Type 30 (8 3/32-inch diameter) = 2 1/2-inch stroke

A brake 1/4-inch or more beyond these limits counts as a defective brake. If 20 percent of the vehicle’s brakes are defective, the vehicle is out of service.

Slack maintenance
Kevin Kreidler, product engineer, slacks and chambers at Bendix Heavy Vehicle Systems, says slack adjusters should be greased every 25,000 miles, three months, or 500 hours of vocational service.

The Gunite service manual specifies six months or 50,000 miles. Use an NGLI grade 1 or 2 grease, and pump it in until fresh grease emerges. On Bendix slacks, it’ll come from the grease relief slit in the rubber boot.

Most slacks now have a provision for greasing the splines where the slack slides onto the camshaft, so you may see grease emerge there. Fresh grease will show up at one place or the other.

Excessive stroke
If the brake stroke is too long, check free stroke. Chock the wheels and release the parking brakes. Measure the distance from the face of the chamber to the center of the clevis pin. Then pry the slack in the direction of application via the front of the chamber and the jamnut on the pushrod until you feel resistance as it begins to apply the brakes. Take the measurement again, and subtract the difference. The dimension should be between 3/8-inches and 5/8-inches. If free stroke is within this range, the slack adjuster is doing its job.
If free stroke is too long, check slack condition and function as described next.

Inspect the basic condition of the slack, making sure it is securely connected to the camshaft (no broken splines) and brake chamber via the pushrod and clevis.

Chock the wheels and release the parking brakes. The basic procedure is to first back off the slack at the manual adjustment hex (turning counter-clockwise). Next, mark the position of the hex in relation to the slack body using a straightedge to scribe a line; or use a rag to hold the wrench onto the hex. Finally, repeatedly apply and fully release the brakes and watch to see if the hex or hex and wrench turn. If there’s no rotation, the slack is not working. Each brand is different in terms of application to bring the slack back to self-adjustment. (Check the manufacturer’s specifications.)

It takes as many as 30 brake applications to bring the slack to full self-adjustment. The best procedure now is to adjust the brakes manually – something that should never be done except after testing a slack, or to get legal when one fails to work. You’ll need to raise the wheel and rotate it while turning the adjusting hex clockwise just until the brakes start to drag. As soon as you feel a slight increase in torque with your fingers, and brakes start to drag, stop. If you don’t have a jack, you can turn the hex until torque increases slightly and tap on the brake drum with a metal bar to make sure the linings are starting to apply and are preventing it from ringing. Then, you back off the hex nut as specified in your owner’s manual – usually 1/2 turn.

Recommendations specified are 1/2 turn for Gunite, Haldex, Crewson-Brunner and ArvinMeritor slacks, and 1/4 for Bendix slacks.

If the slack is working and either type of stroke is excessive, there is a problem within the many parts in the foundation brakes. These problems come from either long-term wear or failure to do a brake job correctly.

For more information, contact:

Bendix Heavy Vehicle Systems
Fax: (440) 329-9557

(248) 435-1000
Fax: (248) 435-1393

(800) 677-3786
Fax (825) 964-0775

Haldex Brake Products Corp.
(816) 891-2470
Fax (816) 891-9447

Crewson Brunner, Inc.
(716) 894-1044
Fax (716) 894-1046

TransAxle Corp.
(800) 257-0444
Fax: (856) 661-0092

The Crewson Brunner Auto-Check pointer indicates proper stroke when brakes are applied. The pointer will index with the green area of the sticker on the side of the slack when stroke is correct, but pass into the red area as stroke becomes excessive.

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