Tractor and trailer air systems may be among the least proactively maintained parts of trucks. Put another way, fleets tend to ignore them until there’s a problem.
There are two serious flaws with this thinking. First, a problem other than an air leak will probably not be noticed until it’s too late. And second, the new annual inspection requirements that debuted in 2014 mandate checking certain systems that never had to be checked in the past. If you aren’t following these new inspection requirements, you’re missing a vital part of the air system inspection – and a valuable early warning.
With that said, air systems are generally pretty reliable and not prone to failures. That can be a blessing and curse. It’s good that they are reliable, but that reliability can lead to a false sense of security. There are plenty of documented cases of air system failures, other than leaks or ruptures, causing calamity.
What can go wrong with a bunch of tubing and few valves? The most obvious is a leak, and leaks are relatively easy to detect and repair. But there are standards to which systems must be maintained, and pressure loss rate is one of them. Depending on the vehicle configuration, the system pressure loss rate cannot exceed 3 to 6 psi over 60 seconds at full system pressure with the brakes fully applied. You may not be able to hear a slight leak like that, especially in a noisy shop, but the reservoir pressure gauge will reveal the leak.
“There’s now a requirement to -perform an air system leakage test on trailers at every annual inspection, as well as to inspect the air tanks, and you have to test the air loss rate,” notes Rolf VanderZwaag, manager of maintenance and technical issues at the Ontario Trucking Association. “Those are specific tests required as part of an annual inspection. These are tests that fleets may not have been doing in the past because they never felt they needed to.” Now it’s captured in National Safety Code (NSC) Standard 11 – Maintenance and Periodic Inspection Standards, October 2014. “These inspections must be done. If not, it’s a violation,” he says.
In addition to leaks, crimped air lines are not uncommon. Unless the crimp closes off the line completely, it could be difficult to detect. Crimps or blockages can slow the delivery of air to the brake actuators, resulting in low application pressure at one or more wheel ends, depending on where the blockage exists.
Air lines tucked into harnesses can also suffer chafing and external rubbing that may compromise the integrity of the line, or result in an Out of Service condition if sufficiently worn when it’s found by a roadside inspector.
These items are all now part of the new annual vehicle inspection procedures found in NSC 11. Section 3A, parts 1-12, include much of the new air system inspection requirements.
In normal service, some problems will not be readily evident. But a proper inspection can reveal them – if you know what to look for. Take, for example, the one-way and two-way check valves and pressure protection valves use to isolate the primary and secondary air systems on tractors since 1975.
“These valves can fail, usually from age and contamination,” says Rick Mello, senior technical services representative at Haldex Brake Products. “If either system experiences a single failure, the [working system] becomes the truck’s -emergency brakes. They must be operationally checked on a regular basis or you risk losing both brake systems during a single failure. This means you would have no emergency brakes.”
Historically, one-way check valves were used between the first air tank [the wet tank] and the two service tanks [primary and secondary tanks]. Some trucks today don’t have wet tanks and use pressure protection valves found in the air dryer to isolate the primary from the secondary.
Double or two-way check valves are also used throughout the brake system to keep the primary and secondary systems separate and independent from one another. Additional pressure protection valves are used to keep auxiliary devices such as air suspensions and air horns from robbing all the air pressure from the brakes during a failure.
“Roadside inspectors can, and sometimes will, check these failure modes by independently draining each reservoir, making sure the other reservoir maintains air,” Mello says. “During these single failures the driver will be instructed to make a service brake application, using the foot valve, and the inspector will check that at least one wheel or axle has executed a brake application.”
Mello advises all heavy-duty maintenance facilities to execute these failure mode tests at least once a year, or at regular Preventive Maintenance intervals.
“If the mechanics are not doing these checks and making sure the two air brake systems remain completely separate, it will never be caught,” says Mello. “If the problem remains undiscovered and one system loses pressure, the other will go down with it rather than remaining at full pressure. That’s the point of isolating the two systems and keeping them independent.”
It starts with the compressor
Compressing moist, humid air creates water in the air reservoirs that have to be drained periodically – daily, in fact – as per most manufacturers’ instructions. However, it’s very difficult for drivers to reach the air tank drain cables on today’s aero-festooned tractors. That means there’s a high likelihood of water making its way past the wet tank and further into the air system. While an air dryer will solve some of that problem, older air dryers with contaminated or oil-soaked desiccant cartridges often are not able to remove enough moisture from the compressed air to prevent significant amounts of water from entering the wet tank.
And since older compressors are also more likely to throw oil, you now have an oil-water mix to contend with, and that can foul the extremely small orifices in some brake system valves, like the expensive ABS valve, and more.
“The extremely tight tolerances on many of the pneumatic servos and solenoids in automated transmissions and the diesel exhaust delivery systems are very sensitive to contamination,” notes Richard Nagel, director of marketing and customer solutions – air charging, Bendix Commercial Vehicle Systems. “Oil in the air tank is the canary in the coal mine. Regular inspection of the exhaust air from the air dryer and the wet tank discharge is a critical maintenance item.”
Most of the air system problems resulting from valve failures can be traced back to contamination of some kind, usually oil, water or dirt, all of which come -ultimately from the compressor, except for possible dirt entering through open gladhand ports.
Air systems are generally pretty -reliable and mostly trouble-free, but the potential for problems still exists. That makes the case for air system maintenance.