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Bad Brakes, Part II
by Jim Park
 We can't overstate the importance of a properly functioning brake system. One aspect of brake system function consistently gets a lot attention - brake adjustment - while what goes on upstream of the slack adjuster is seldom talked about. Since it's usually the driver left holding the smoking gun after an accident, you've got as large a stake as anyone in ensuring the whole system is working properly, and that includes balance and timing. Balance, timing, and brake system plumbing are just as likely to be factors in accidents as adjustment, but are almost completely ignored, both at roadside and on the shop floor. "Unfortunately, the industry has been led into the 'brake defect' trap by the relentless enforcement of brake stroke at the expense of the rest of the system - especially the defects that can't be detected on a visual inspection," Dale Holman says. He owns and operates a brake service facility called Truck Watch Services in Georgetown, Ont. In the report recently issued by the Ontario Trucking Association, Improving Trailer Brake Safety, author Rolf VanderZwaag describes several incidents where brakes appeared to be working properly, but little if any braking force was actually applied at the wheel end. The reason, in several of the examples, was the installation of an incorrect relay valve somewhere in the system, or improperly plumbed relay valves. It's easy to confuse one valve for another, and mechanics and parts handlers
may be mistakenly installing incorrect valves. There are issues with valve interchangeability,
as well, as some manufacturers suggest their valve is a suitable replacement
for X valve, when in fact there are differences, perhaps improvements, in the
design that cause the valve to function in a slightly different manner. As VanderZwaag points out in his report, the need for more comprehensive training for technicians and drivers is clear. The drivers' role should include a better understanding of how the systems work so they can provide a more accurate description of the problem. Technicians need to be able to identify faulty valves, and then be able to identify and install the correct replacement, and to perform a basic brake system diagnostic check. While there's little the driver can do about the problem at the technical
level, there's a great deal to be done on a daily basis in recognizing
potential problems before they become tragedies. While this may be a training
issue for repair shops and fleet maintenance technicians, the consequences of
their mistakes might be yours alone to deal with. Once we've got the air going where it's supposed to go, there's still the question of how much brake force is actually produced at the wheel end. Ideally, you want all the wheels, except the steers, doing an equal share of the work. You want an equal amount of force applied to each slack adjuster, an equal amount of lining-to-drum contact, and an equal amount of stopping force generated by each brake. Unfortunately, there's no practical way of measuring brake application force at a roadside inspection. Testing of this nature is rarely done in the shop either. Holman says brake balance is a real problem on most of the trucks he tests on his roller brake dynamometer, a device used to measure balance, timing, and other brake system functions. "It's not unusual to see twice the amount of force applied on one side of an axle compared to the other," he says. "And that can create some real vehicle handling problems, particularly when lightly loaded or in a high-application-pressure panic stop." Mismatched lining/drum combinations, contaminated or glazed linings, slack adjusters of different lengths, variations in the diameter of the air lines between service tank and the brake chambers, different sized chambers, etc. - the list can be long, and as you've probably seen yourself, none of these conditions, or combinations of conditions, is at all rare. In troubleshooting for balance, drivers should be aware of any pulling to the
side when stopping or individual wheels that run hotter or cooler than the others.
Visually, it's absolutely vital that all components on the same axle be identical,
and ideally, all brake components on the vehicle (truck or trailer) should be
the same type and brand. Report brake chambers or slack adjusters of different
sizes or lengths, and any additional fittings or hoses that may be present on
one brake or axle that aren't fitted to the other axle(s). With balance and timing, you can't see the problem, but you can sure feel it. It's important to communicate the problem to the mechanics in such a manner that they understand there might be more required than just a brake adjustment. And while we've still got the ball, we need to be firm in refusing any request to move an unsafe trailer anywhere. Next month, we'll delve into the murky water of excessive pushrod travel
and brake adjustment: why is the stroke limit so critical, why is it so hard
to keep brakes properly adjusted, and why is so much emphasis placed on brake
adjustment? Common But Concealed Brake System Defects Common But Ignored Wheel-end Defects
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Most repair shops
don't have the brake testing equipment to diagnose problems that aren't
obvious on a visual inspection, and drivers' observations and descriptions
of the problem often aren't accurate enough to prompt a thorough investigation
of the complaint. Telling the mechanics that the brakes aren't working
very well is a start, but were a couple of brakes found to be beyond their stroke
limit, the investigation would likely go no further. They would be adjusted
and the trailer put back in service. 
You also
want all the brakes to apply and release at the same time, or within fractions
of a second of each other, as required by federal standards. Relay valves can
affect brake timing, as can the length and diameter of the brake hoses. A 90-degree
elbow fitting can slow the delivery of air to a brake chamber considerably,
thus upsetting the balance and timing of that wheel. Timing tests, like brake
application-force tests require special equipment as well, and therefore are
rarely done. 
