Priority Report: Trailer Brakes

Avatar photo

Most incidents involving total air-brake failures go unreported officially and simply become the stuff of legend. But a few too many of them have resulted in very public collisions. Until recently, accident investigators just didn’t have the tools to probe a truck crash deeply enough to reveal problems involving service brakes. Frankly, the time and cost involved in conducting a thorough investigation weren’t always warranted. A jackknife was a jackknife, probably due to driver error, end of discussion. Truck didn’t stop in time and popped the Honda ahead of it? Conclusion: the driver must have been following too close.
As long as the brakes were adjusted properly, investigations into bad accidents usually stopped with the driver.

But a recent fatal collision on Hwy. 427 in Toronto did prompt a more thorough look-see-using a high-tech device called a performance-based brake tester, or PBBT. The brakes on six out of nine axles were not working when a tractor-trailer sped into an intersection at the bottom of an offramp. After striking several vehicles, the rig continued another 1,200 feet before finally stopping.

“I think about that crash quite a bit,” says Rolf VanderZwaag, maintenance director for the Ontario Trucking Association. “I think about how fast that vehicle was travelling 280 feet after the driver hit the brakes. Two-eighty is the U.S. stopping-distance mandate. I’m going to guess that nine-axle tractor-trailer, with all that weight behind it, was going around 40 miles an hour when it passed the 280-foot mark. There was nothing that driver
could do to stop. He just kept right on going. At 1,200 feet, that’s about five times the required stopping distance.”

VanderZwaag says the driver probably had no warning that his brakes wouldn’t properly apply. The spec met CMVSS 121, the federal regulation that governs truck and trailer brakes (it mirrors U.S. FMVSS 121 in the United States), and mechanically everything appeared to be sound. As he cruised down the highway, there would have been no noticeable change in the way the vehicle was handling, no alarm buzzer going off, no feeling of panic until after the driver stepped and then literally stood on the pedal as he rumbled toward the stop lights at the base of the ramp.

What happened? VanderZwaag points to a flaw he says is inherent in trailer spring-brake priority systems that can be traced back to 1992. That’s when the U.S. National Highway Traffic Safety Administration made two regulatory changes designed to reduce the chances of dragging trailer brakes, which the agency says can lead to trailer fires: the elimination of a requirement for a separate trailer reservoir dedicated to releasing spring parking brakes, and the addition of a trailer protection valve.

The protection valve keeps the trailer air pressure from dropping below 70 psi, in theory keeping the spring brakes (which require 60 psi) from engaging even if the trailer’s service reservoir loses air. The reality, VanderZwaag notes in a discussion paper called Improving Trailer Brake Safety, released by the OTA in June, is that this setup can allow a partial or complete loss of service brakes on the trailer to happen.

A spring-brake priority system routes supply air to the trailer’s spring brakes immediately on hookup so you can release the parking brakes quickly. This is handy when the emergency brakes are applied on a unit and it’s in a precarious position, like inside a tunnel, or it’s on fire. It’s also nice if you’re shunting trailers around or waiting in line and you have empty tanks. The volume of air flowing into the reservoirs is relatively small, so bringing an empty air system up to full pressure can take several minutes. The reduced airflow is the result of that requirement to not allow the spring-brake air system to fall past 70 psi; if the flow were greater, the outflow from the supply line to the reservoirs would permit the pressure to drop below that 70-psi threshold.

The unintended consequence here is that the tractor low-air warning system won’t detect even a serious air leak in the trailer’s service system. The pressure protection valve designed to protect the spring-brake system will maintain a minimum of 70 psi in the parking-brake chambers and the supply line. Beyond that, any air from the tractor could be just venting into the atmosphere as a result of an air leak in the service system, or even a ruptured air tank or a severed line.

The air from the tractor that’s trying to make up for the loss must first pass through a relatively small orifice in the spring-brake control valve, which helps keep the supply line air pressure-and the tractor air tank pressure-above the point that would trigger a low-air-pressure warning in the cab, somewhere around 60 to 65 psi.

As an example, VanderZwaag cites the loss of trailer service brakes due to a failed air tank. A drain valve broke away and the air bled out of the tank, reducing the pressure in the trailer supply line and causing a draw on the tractor air supply. Since the pressure-protection valve in the trailer’s spring-brake control valve restricted the rate of air loss through the open air tank, the air compressor was able to maintain the demand for air from the trailer. The air pressure in the tractor dropped to slightly less than 100 psi. The trailer spring brakes were held off by means of the pressure protection valve, and the vehicle remained stable in this condition.

When the driver applied the service brakes, the control signal sent to the trailer opened a balancing valve that tried to draw air from the failed tank. Since there was no air to send to the subsequent relay valves, and there was no means for air to flow on through the valve, the brakes failed and the vehicle crashed.

Problems with air-brake systems are more acute in Canada, where half the trailer population has three or more axles. In the United States, the number is just 3 per cent. “U.S. 121, which we’ve basically adopted and to which our equipment conforms, is written for the United States,” VanderZwaag explains. The needs of multi-axle trailer configurations-which are heavier, require more plumbing, and take longer to stop-just aren’t on the radar screen.

They should be. Say you’ve got a six-axle trailer behind a three-axle tractor. If you proportionately drop the amount of brake force on six out of nine of those axles, when those brakes are cold, with just the tractor brakes working, you have 22 per cent of your braking force, VanderZwaag says. “You’ll need 1,258 feet to stop. And with only those tractor brakes working, they are going to get hot very, very quickly, so now say you’re down to 15 per cent. Your stopping distance is out there around 1,800 feet.” Provided you’re not slowed by any sort of impact with cars at the bottom of a freeway offramp.

It’s hardly news that the flip side to being able to move a trailer immediately is that the service brakes may not yet be working when the driver pulls away. That’s more a flaw in the way drivers have been trained to do their job, something VanderZwaag and other brake experts across Canada have talked about before.

It’s easy to accept that mechanics, too, need better training. In the current 720-hour curriculum given to apprentice truck repair technicians in Ontario, less than 30 hours are devoted to air brakes. Air brakes aren’t complicated, VanderZwaag explains. Once you know what to look for, the repair is easy. Unfortunately, technicians don’t diagnose problems well. Instead, they are wont to swap components, and then fail to properly test and confirm that the repair has been done successfully.

A lot of valves look alike, which doesn’t help those technicians who are borderline-qualified to begin with.

“You’d better have a binder with a schematic of every vehicle in your fleet in the parts department or in the shop,” he says, “so the technician can look at that thing and say, ‘Unit so-and-so gets this valve, this valve, and that valve.’ Because lots of guys are putting on the wrong ones.”

VanderZwaag will get no argument that drivers and technicians need better training. But in his paper, he calls for significant changes to federal braking standards for trailers with three or more axles. He calls for dual service systems: this could involve three air-line connections between the tractor and trailer, and modifications to the tractor. Use of dual control relay valves would ensure that if one circuit failed, control signals would still get through. All relay valves should have back up air sources or other means of redundancy.

He says the tractor protection system should have a low-air warning system that’s set to 95 psi to catch problems that wouldn’t otherwise show up on the tractor’s air pressure gauges.

Furthermore, the driver needs an in-cab alert when the trailer air system has low air pressure. This requirement would have gone through with NHTSA’s 121 revisions in 1992 but didn’t because industry lobbyists feared it would cost too much. “Guess what happened about six years later? NHTSA required the light for ABS,” he says. “So we refused a warning to tell the driver he may not have any brakes, but it’s there to say he may have an ABS problem but not to worry because he still has normal braking. It’s absurd.”

VanderZwaag argues that provincial safety inspections should involve more intensive checks of braking systems. But that would require sufficient knowledge on the part of technicians to actually inspect the system, he says: “Obviously, you can’t have high inspection standards that nobody can apply. Changes to the way we approach training and inspections need to go forward together.”

He suggests that vehicles be required to undergo periodic performance-based brake testing. Make it part of the fleet’s annual safety check. Or every six months. “You’ll learn things about what you should be doing different that will pay dividends that more than cover the cost of the testing,” he says.

VanderZwaag adds that there’s been a reluctance to modify existing systems in Ontario, where multi-axle units are common, because even though the improvements would be welcome, language in the province’s Highway Traffic Act leaves open the interpretation of “modification.” There may be liability issues for fleets who do conduct modifications that aren’t consistent with existing federal and provincial standards. On the positive side, he says the advent of multiplexing has opened new opportunities for brake system designers to help technicians and drivers monitor their brake performance.

It would be a welcome change for an industry where driver complaints about brake problems are dismissed at a quick glance to the hardware, where technicians sometimes feel at a loss as to how to troubleshoot what the boss says is a dead-easy system, where it’s easier for enforcement agents to grab a stroke gauge and violation book than to measure a truck’s ability to stop, and where the people writing the rules know far less about the intricacies of the hardware than they should.

“Everyone knows now that the trucking industry is working to come up with a solution for these problems,” VanderZwaag says. “From a due-diligence point of view, we believe this is the right approach. We really want these problems resolved.”

Avatar photo

Jim Park was a CDL driver and owner-operator from 1978 until 1998, when he began his second career as a trucking journalist. During that career transition, he hosted an overnight radio show on a Hamilton, Ontario radio station and later went on to anchor the trucking news in SiriusXM's Road Dog Trucking channel. Jim is a regular contributor to Today's Trucking and, and produces Focus On and On the Spot test drive videos.

Have your say

This is a moderated forum. Comments will no longer be published unless they are accompanied by a first and last name and a verifiable email address. (Today's Trucking will not publish or share the email address.) Profane language and content deemed to be libelous, racist, or threatening in nature will not be published under any circumstances.