by Jim Park
Ever had one of those
hair-raising moments when you hit your brakes but next to nothing happens? It
will be of little comfort to you now, but that's not as rare as you might think.
It seems there's a mighty serious design flaw - as the direct result of an official
American safety mandate, believe it or not - in so-called spring-brake-priority
trailer air systems. And it means that a tractor's low-air warning system won't
necessarily detect even a serious air leak in the service system.
Most incidents involving such total failures likely go unreported, but a few
too many of them have resulted in very public collisions. And there's
been at least one fatality reported in the Toronto area resulting from a documented
case of trailer service-brake failure. Based on the number of trailers now being
discovered with problems involving service-brake functionality, it seems likely
that there have been other incidents where failed service brakes were a factor.
We have the Ontario Trucking Association (OTA) to thank for bringing this situation
Until recently, accident investigators just didn't have the tools to probe
a truck crash deeply enough to reveal the real source of the problem, and frankly,
the time and cost involved in conducting a thorough investigation weren't
always warranted. A jackknife was a jackknife, and probably due to driver error,
end of discussion. Truck didn't stop in time, driver must be following
As long as the brakes were adjusted properly, investigations usually stopped
with the driver. But a recent fatal accident did prompt a more thorough look-see
- using a performance-
based brake tester, or PBBT. Results of the after-the-fact tests revealed that
the crash was the
direct result of mechanical deficiencies in the braking system that would have
been virtually undetectable until the brakes were applied.
So after all these years, we're finally getting beyond brake adjustment
and presumed driver error, and looking at the brake system in its entirety;
we're finally trying to understand how one aspect of the system affects
the function of other components downstream.
The OTA has just released a discussion paper called "Improving Trailer Brake
Safety" outlining these brake-system deficiencies. It's trying to get everyone
involved - origi nal equipment manufacturers, aftermarket suppliers, technicians,
Transport Canada, the U.S.
Department of Transportation, and various arms of the enforcement community
- to look at the issue. The idea is to work together and come to some practical
solutions. According to the OTA's maintenance and technical issues manager,
Rolf Vanderzwaag, proper analysis of that one fatal accident and subsequent
testing of other vehicles has revealed a flaw in the functionality of some trailer
brake systems. It's been found that a partial or complete loss of trailer service
brakes can occur on trailers configured to current requirements, and there will
be little if any perceptible change in vehicle handling or operation beforehand.
"Since the loss or compromising of the trailer service brakes isn't noticeable
in the cab, the problem generally occurs with no warning to the driver until
he needs his brakes," Vanderzwaag says.
What to Watch For
Experienced drivers can feel when something is 'not quite right' down below,
but their complaints are sometimes taken less than seriously, particularly when
adjustment and functionality appear to be normal. But if something really doesn't
feel right, you should report the symptoms as accurately as you can (air-pressure
readings, sounds, responsiveness, operating conditions like road speed and engine
rpm, etc.) to your mechanic and let the shop have a go at it. If you're not
comfortable with it, don't drive it.
Tell-tale signs of a potential problem include:
• tractor air-pressure gauges read less than normal but don't trigger
a low-air warning;
• diminished brake performance;
• brakes appear to work fine one minute but not the next, then they feel
• truck pulls to the right or left during a hard application;
• trailer bumps against the fifth wheel during a brake application;
• brakes don't release immediately once your foot's off the
• certain wheel positions have more brake dust on the rims or run hotter
Here's how to check for brake-system deficiencies:
• leave the vehicle, walk around and listen for air leaks, preferably
with the wheels chocked, the
engine off, the transmission in gear, and the brakes released;
• do the same inspection with the brakes applied;
• conduct a thorough brake test every day, per the instructions in the
• do frequent walk-around checks, smelling the brakes or touching the
drums; if some are noticeably hotter or cooler than the others make note of
the wheel position(s) and report your findings;
• any timeafter you've run over some debris on the road, give the
trailer-brake valve a tug to test brake action; when stopped, make a full brake
application and listen for leaks;
• watch for individual wheels that lock up before the others, or don't
lock up when the others do.
Emergency procedures -- if you ever discover you've lost trailer
braking capability, you likely won't have time to react properly anyway,
but it might help to quickly reach down and pop the red button out. Closing
the trailer supply valve should cause the trailer spring brakes to engage almost
Drivers are often told never to do this, so most of you have never experienced
an emergency springbrake application. The spring brakes produce the equivalent
of a 60-lb brake application. If the truck is loaded, the wheels usually won't
lock up, though an empty trailer probably will lock up if the spring brakes
are working. You could try an emergency stop sometime, under safe conditions
like a quiet side road at a safe speed of 20 mph or so. If the spring brakes
take a long time to apply, get the trailer to the shop.
The first word in emergency procedures is always leaving yourself an out. As
your mind is grappling with the fact that you're not stopping even though
you're standing on the treadle valve, remember, you might be able to steer
around the obstacle. Good luck, amigo.
Spring Brake Priority
It's important to note that we're talking about equipment built to Canadian
Motor Vehicle Safety
Standard 121 (CMVSS 121), which applies to all trailing equipment manufactured
in Canada. It mirrors the U.S. rules contained in FMVSS 121, first introduced
in 1975. That 28-year-old standard for brake-system functionality worked when
it was introduced, and still does under
ideal circumstances. Trouble is, the requirements for protecting certain air
circuits in trailer-braking systems were compromised in 1992 when the U.S. National
Highway Traffic Safety Administration (NHTSA) introduced changes to FMVSS 121.
Those changes, combined
with a few fatal flaws in the original requirements that weren't fully understood
until recently, can
seriously reduce trailer-braking capability under certain conditions.
The problems discussed in the OTA paper revolve around the socalled spring-brake-priority
trailer air systems. The problem is that the systems are designed, after first
hooking up and charging the trailer with air, to direct the airflow to the springbrake
chambers first. Why? To release the parking brakes quickly.
Seriously - and this is the crux of the problem - one of the concerns
NTHSA took to heart when he agency was contemplating changes to FMVSS 121 in
the early 1990s was a request for rapid release of the parking brakes to reduce
time in trailer shunting operations.
Once the pressure in the spring brake chambers reaches 70 psi (formerly 55
psi under the old FMVSS 121 rules), a bypass valve opens and allows air to flow
into the trailer air tank(s). The volume of air that flows through the bypass
valve 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 the requirement that the spring-brake air system not be permitted to drop
below 70 psi. If the flow were greater, the outflow from the supply line to
the reservoirs would permit the pressure to drop below 70 psi.
The other reason for prioritizing the airflow to the spring-brake chambers,
and for protecting the spring-brake air system from losing air to the service
system in the event of a leak, is to prevent the spring brakes from applying
with no means of releasing them quickly in an emergency. As long as there is
sufficient air pressure in the supply line, the brakes can be released.
The unintended consequence to these springbrake- priority systems is that the
tractor low-air warning system won't detect even a serious air leak in the service
system. The valve designed to protect the spring-brake system (the pressure
protection valve) will maintain a minimum of 70 psi in the parking-brake chambers
and the supply line. Beyond that, any airflow from the tractor could be just
venting to atmosphere as a result of an air leak in the service system (even
a large leak like a ruptured air tank or a severed air line). But 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 (60-65 psi) that would trigger a low-air-pressure warning in the cab.
Here's an example of just such a failure, from "Improving Trailer Brake Safety.
"Condition 1 - complete loss of trailer service brakes caused by an air tank
failure, in this case involving a 1997-model trailer on which an airtank drain
valve broke away. The air drained 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
in accordance with CMVSS 121 S5.8.3, and the vehicle remained stable in this
condition. No indication of the problem was given to the driver.
Often referred to as brake dynamometers, these machines check and record brake-system
performance using air-pressure sensors, timing devices, and a pair of rollers
that measure braking force applied to each wheel. The complexity of the brake
system and exacting brake-performance standards demand an accurate way of determining
functionality and performance, well beyond what a technician can determine visually.
When used as a diagnostic tool, PBBTs can observe and record the opening and
closing pressures of various valves within the system that affect application
and release timing; the air pressure delivered to the brake chambers during
a brake application; and perhaps most importantly, the braking force that's
felt between the wheel and the pavement. PBBTs can also measure and compare
an individual brake against its mate at the other end of the axle, and to the
others on the truck. When PBBTs are used in accident analysis, components that
survive the crash can be tested for functionality and performance. The results
will reveal any system failures or deficiencies that may have contributed to
the accident. Accidents once thought to have been due to 'driver error'
just might, after analysis, turn out to be the result of a faulty brake system.
And now there's talk about using PBBTs as an enforcement tool to measure
brake performance during a roadside test. Almost everyone agrees, though, that
it would be premature at this point, given the type of defect they're
capable of revealing and the apparently slim possibility of finding a perfectly
functioning brake system.
"The industry is more than a little concerned about the use of a PBBT at roadside
inspections," says Dale Holman, president of Truck Watch Services in Georgetown,
Ont., one of a handful of people in Canada using PBBTs as part of a brake inspection
and maintenance program.
Holman hopes the enforcement community will not latch onto the PBBT as a cash
cow, rather use them to their true advantage in making the roads safer by uncovering
previously invisible defects.
"They could start grounding trucks right, left, and center because they
fail to meet standards, but without providing any insight into what needs to
be repaired, the exercise won't accomplish anything positive," he
says. "That would be the wrong approach. Industry needs some time to move
away from the brake-adjustment-is-all-that-matters mentality, and move toward
a more comprehensive approach to brake-system performance measuring.
When a service-brake application was made, 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, no service-brake action took
place. This vehicle was involved in a serious crash. Searching for Solutions
The difficulty in achieving a solution is getting everyone to agree there's
a problem in the first
place. The PBBTs are shedding new light on this issue, thanks to the few post-accident
investigations that have taken place. There still isn't a large body of
evidence that can be referred to, but the fact that drivers have reported mysterious
brake failures for years, coupled with the results from the PBBT testing, make
the issue impossible to ignore. The next step is to convince the regulators
that their rules are causing problems, but the OTA and their brake experts already
have the attention of Transport Canada and the U.S. DOT.
That's a start. Several corrective actions for this particular issue
have already been floated, including the use of lowpressure warning systems
on trailers, dual-circuit air systems for trailers, raising the low-pressure
warning threshold on tractors to 95 psi so the system could sense a loss of
trailer air pressure, redundant air tanks as a back-up air supply for the service
system, and revisions to the valving requirements that would allow the spring
brakes to apply in the event of a service system failure backed up with another
air tank for an emergency brake release.
Vanderzwaag suggests that the improvements could be made at the manufacturing
would add less than $100 to the price of a new trailer. "Corrective add-ons
and modifications to
existing equipment should still cost less than $500, including shop time,"
he says. But fixing this particular problem is only the tip of the iceberg.
It has, though, revealed flaws in the way we've been doing things all
these years. It's shown the need for more comprehensive training for brake
service technicians, for example. It has illustrated the need for more commonality
between different brands of air valves, and the need for easier valve identification
and interchangeability. It has also established the need for better test procedures
for trailer brake systems to be used in a preventive maintenance program. And
that's not all.
Other recommendations contained in the OTA paper suggest we should:
• establish 'best practice' guidelines for spec'ing,
inspection, and repair of trailer brake systems;
• conduct more intensive inspection and testing of trailers involved in
• enact provincial standards for air brake inspection requirements, including
periodic testing with PBBTs;
• establish standards for licensing of brake technicians;
• and provide more air-brake awareness training for drivers, particularly
in regard to performing daily inspections and emergency procedures.
There are other concerns about braking systems that haven't been addressed
in the OTA paper, but they will no doubt be coming up for discussion in coming
months. We'll certainly be looking at a few of them in the September issue
Vanderzwaag says many of the faults that may lie within a tractor-trailer's
brake system are difficult to pinpoint. He adds that there's been a reluctance
to modify existing systems in Ontario, even though the improvements would be
welcome, because of certain language in the province's Highway Traffic
Act that 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. He says the time has come to
address these issues and, once and for all, to establish operational, testing,
and repair/replace guidelines and requirements that serve the industry rather
than hurt it.
"Everyone knows now that the trucking industry is working to come up
with a solution for these problems," he says. "From a due diligence
point of view, we believe this is the right approach. We really want these problems
A copy of the OTA report, "Improving Trailer Brake Safety," can be downloaded
from the OTA website, www.ontruck.org,