Don't Let Poor Brake Adjustment Catch You Unawares

Brake adjustment is a pain, but it doesn’t have to be if you use stroke indicators.

April 2013, - WebXclusive

by Jim Park, Equipment Editor - Also by this author

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Everybody worries about getting a ticket for having badly adjusted brakes, but a ticket would be preferable to the other possible outcome. While equipment defects are a cause in only a small percentage of truck crashes, brake defects account for a large percentage of the trucks taken out of service. Many of those defects are adjustment issues.

Yes, marking and measuring brake stroke is a grand pain in the posterior, but it’s technically part of the trip inspection. If you sign the trip inspection form, you’d better be sure the brakes are in adjustment.

There are a couple of so-called short-cut methods for determining brake adjustment, but they’re far from accurate enough to pass muster with DOT inspectors. Those guys mark and measure brake stroke right down to an eighth of an inch, which doesn’t leave you much of a margin for guesswork.

You could release the spring brakes and build the system pressure to 100 psi, make a full pressure application and note the pressure drop. Some say a 2-psi per axle drop in system pressure (10-psi on a 5-axle unit) is acceptable. It may be, but how do you know you haven’t got two brakes adjusted well below the limit and three over-stroking by an eighth of an inch? You can’t tell, but the creeper cops can.

Some say that if the slack adjuster is at no more than a 90-degree angle to the push rod when the spring brakes are applied, the adjustment is okay. In the same vein, many believe that if, when using brake stroke indicators, the pointer is inside the markers when the spring brakes are applied, the adjustment is okay. Neither is true, and neither of these methods is an accurate way of measuring brake stroke.

Constant Adjustment

Slack adjusters, as the name implies, are designed to take up the slack in the stroke as friction surfaces wear away. In “normal” service in a U.S. highway environment, a manual slack adjuster might need adjustment every 10,000 miles or so, says Dale Holman, president of TruckWatch Services in Georgetown, Ontario.

“But there’s no such thing as normal when you’re talking truck brakes, and that’s why slack adjusters are designed to be adjusted. You make the adjustment when it’s required, not because some mileage interval has come and gone. It’s not like an oil change.”

Manual slack adjusters require operator intervention, and could get close to the adjustment limit in a short time under certain conditions. Automatic slack adjusters do an admirable job overall of keeping stroke within limits, but a lot of auto slacks are found to be out of adjustment during roadside brake inspections. With manual slacks, improper adjustment is often a matter of negligence or laziness on the part of the operator, but when auto slacks go out of adjustment, there’s usually something else going on.

If you take nothing else away from this piece, remember this: Automatic slack adjusters normally don’t require manual readjustment. If you have a brake that is over-stroking and it has an automatic slack adjuster, you have a problem with either the brake or the adjuster. If you readjust it, you aren’t really fixing the problem. A manual readjustment may bring the brake back into compliance and improve the way the brake operates, but it will only be temporary.

Faulty auto slacks can be a factor, Holman says, but that’s rare. “If there is a mechanical issue, it’s more likely to be improper initial installation, or faulty or worn components elsewhere in the brake system.

“But in my experience, the primary cause of auto slacks stroking beyond their limit, is – believe it or not – good drivers,” he says. “I’m talking about the driver who never makes an application harder than 15 or 20 psi because he or she never has to. They’re the ones who manage speed well, keep a safe distance, and coast up to traffic lights. These drivers hardly ever put enough torque through the adjuster to cause the ratchet to roll over to the next peg. Consequently, as the brakes wear naturally, the auto slacks aren’t compensating.”

What these drivers need to do is make half a dozen full pressure applications once a week or so to get the adjuster to turn over, and then visually check the stroke before leaving the yard.

Next page: Brake stroke indicators

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