Fire Fighting in Canada

Truck Checks: Air-brake maintenance

There are a lot of myths, rumours and superstitions about the maintenance and operation of air-brake systems. First, let's get the math out of the way. The engine may be required to accelerate the truck to 100 km/h from 0 km/h in 30 seconds.

December 6, 2007 
By Don Henry

are a lot of myths, rumours and superstitions about the maintenance and
operation of air-brake systems. First, let's get the math out of the
way. The engine may be required to accelerate the truck to 100 km/h
from 0 km/h in 30 seconds. To accomplish this may require 200
horsepower from the engine. That same truck may be required to slow
from 100 km/h to 0 km/h in just six seconds; this would require the
brakes to absorb more than 2,000 horsepower. The brakes accomplish this
by converting the forward energy of the vehicle to heat through
fiction. This heat causes all kinds of problems. A highway truck may
use its brakes only every few hours, as the driver knows where the next
rest stop is and can begin to gear down before the stop, thus using the
brakes less often and extending their operational life. Fire apparatus
often make many aggressive stops. This causes the brake lining material
and the brake drums to overheat. This heat has two effects; first, the
drums expand and get larger and, second, the brake material losses its
coefficient of friction. When this happens, the brake lining does not
grab the drum as well. This condition is referred to as brake fad and
it causes the truck to not be able stop as fast. The first few stops
out of the firehouse are very normal because the brakes are still cold
but as the brakes overheat the truck will not stop as quickly. This
next bit of information is just plain physics: If you double the weight
of the truck you will need to double the stopping power. If you cannot
double the stopping power then you will need twice the distance to
stop. This is just one reason we should not overload the apparatus. The
real problem comes with speed. If the speed of the vehicle is doubled
then the stopping force will increases by a factor of four. If both
weight and speed are doubled then the increase is by a factor of eight.
While we can not do much about the laws of physics we can take measures
to ensure the truck can stop.

1. Know the route; do the planning and understand the grades of the hills you will need to deal with.

Ensure your brake-slack adjusters are correctly set with regard to
angle and adjustment. As the brake drums heat up, they expand. If the
brake-slack adjustment was marginal when the brakes were cold, it may
be out of adjustment when the brakes are hot. This is probably the
greatest cause of air-brake accidents. Make all your drivers/operators
complete an air-brake course and ensure they know how to inspect the
slack adjuster for correct adjustment and operation. It may be your
department's policy to allow only qualified emergency-vehicle
technicians to adjust the brakes and that's all right, but the driver
must be able to recognize when the brakes need adjusting.

When coming to a stop, do not pump the brake pedal (also called fanning
the brakes). This wastes compressed air. It is better to make a steady
brake application. Myth one: You can pump up the air brakes. This is a
holdover from hydraulic brakes. The first application of the brake
pedal will give the most amount of air pressure. Any additional pumping
of the brakes just wastes valuable air pressure. To make sure the truck
can stop repeatedly and not suffer from brake fad, the truck may need
an auxiliary braking device. NFPA 1901 says all apparatus with a gross
vehicle weight rating of 36,000 pounds (16,330 kg) or greater shall be
equipped with an auxiliary braking system. You could, of course,
install the auxiliary-braking devices on lighter weight units.


Drain air tanks daily
contains moisture. This humidity is measured as a percentage and it
affects the air-brake system. Moisture-laden air will cool in the
truck's air tanks. This moisture will settle out and collect in the
bottom of the tanks. Water, of course, is not compressible and will
accumulate in the air tanks of your braking system. It needs to be
drained out. If it is not drained then the effective size and
capability of the air system is reduced and less air available for
aggressive stops. The second problem will come during cold-weather
operation. The water in the tanks, lines and valves will cause the
brakes to malfunction and freeze up. You can ensure this does not
happen by draining the air tanks daily. Drain the supply tank first,
then the primary and secondary tanks. This order will ensure that
excessive contamination in the supply tank does not get forced into the
other tanks. The senior personnel in your department may have called
the supply tank the wet tank. Myth two: You don't have to drain the air
tanks because you have an air dryer. Wrong. Air dryers are great
devices and I recommend them on each truck I help departments spec out.
An air dryer ensures that moisture does not reach the air tanks. But
air dryers do fail, even though they may be purging when the air
compressor cuts out (that's the bush sound you hear), this does not
mean the dryer is removing the moisture from the air. The air dryer
needs its desiccant cartridge changed regularly. How often? A cement
truck in a major city may need the desiccant changed every six months
to one year. A fire truck could go longer without a change, it just
depends on the amount of use. It needs to be changed when the daily
draining of the air tanks reveals water in tanks.

Compressor build-up test
air-brake system was sized according to the Canadian Motor Vehicle
Safety Standard 121. This standard determines how large the compressor
needs to be and the size of the air tanks. I have noticed that some
departments have allowed this test to slip over the years. It is very
important for the following reasons: NFPA 1901 says "a quick build-up
section in the air-reservoir system arranged so that if the apparatus
has a completely discharged air system, it is able to move within 60
seconds of startup. The quick build-up system shall provide sufficient
air pressure so that the apparatus has no brake drag and is able to
stop under the intended operating conditions following the 60-second
build-up time. On a chassis that cannot be equipped with a quick
buildup air-brake system, an onboard automatic electric compressor with
an automatically ejected electric shoreline or a fire station
compressed-air shoreline hookup shall be permitted in order to maintain
full stand-by air pressure while the vehicle is not running." In other
words, the truck needs to go from no air pressure, out the door and to
a full stop at the end of apron before you reach the street. Think your
truck can do that? I have never been a big fan of onboard electrical
compressors or the use of shorelines. I feel these give a false feeling
of security – you think the air system on the truck is operational just
because the building's air compressor is functioning. If you are going
to use a shoreline, make sure someone in building maintenance is
draining the water from the building's air compressor. Often, the
building's maintenance department thinks the fire department is
draining this moisture. Another problem is that many onboard air
compressors don't remove moisture from the air before it is pumped into
the air tanks. These onboard compressors do not route through the
truck's air dryer and may be continually pumping in moisture-laden air.

Combine this with failure to drain the air tanks daily and you very quickly
get an air-brake system that is full of water. The compressor build-up
time also gives us much more information. If the time is too fast, it
may be because the air tanks are full of water, thus reducing the
volume. If the time to build up is too long then there may be an air
leak in the system or a worn air compressor. A common method of
checking compressor build-up time is as follows: With the engine cold,
time the air buildup of the tanks from 50 psi to 90 psi. This should be
done with the engine running at 1,200 r.p.m. The system should build up
pressure in less than three minutes and, in many cases, a lot less then
three minutes.

Let's say your truck takes one minute, 30
seconds and has done so from the day it came from the manufacturer. If
today it takes two minutes and 30 seconds, I would want to find out
why. Besides air leaks, one common cause is worn compressor piston
rings. If the compressor piston rings are worn then the compressor will
pass oil into the air tanks and be slow to pressurize the tanks. Of
course, you will catch that with the daily draining of the air tanks. A
small amount of this oil is normal.

Excess amounts will foul the air
dryer. Warning: Make sure you wear eye protection when draining the air
tanks – any contamination can exit the drain at a very high velocity
causing eye damage. If the tanks have not be drained for years (Don't
laugh, I have seen it!) then the drains maybe plugged with rust or
scale. Don't use a device like a nail to make a hole in the drain. The
compressed air could drive the nail into your hand. I often use an old
coat hanger. This works well (and the world has too many coat hangers
anyway!). If you continue to get rust and scale from the tanks, the
tanks need to be replaced.

Anyone who has taken a basic
air-brake course should know everything covered above, with the
exception of the NFPA 1901 standard. I am amazed that we have people
who operate fire apparatus with air brakes who have not taken an
air-brake endorsement course. Oh yes, and before I forget, did I
mention that it's very important to drain your air tanks daily. In
future articles we will cover anti-lock brakes and adjustment of slack

Print this page


Stories continue below