The air reservoir (air tank) function is to provide
a volume of compressed air used in braking the vehicle.
There are at least three reservoirs on trucks with
the FMVSS 121 brake system. These tanks are supply
reservoir and secondary and primary reservoirs. The
primary reservoir is the air source for the rear brakes and
the front brakes are supplied by the secondary air tank.
In some instances, the supply reservoir and secondary
reservoir may be in the same tank with a separation
inside the tank (Fig. 1).
Another function of a reservoir is to provide a
place where the air, heated during compression, may
cool and cause the oil and water vapors to condense.
The combined volume of all service reservoirs
and supply reservoirs are 12 times the combined volume
of all service brake chambers at maximum travel of the
pistons or diaphragms. The size of air tanks should
never be altered without IH Engineering approval.
The reservoirs should be completely drained
daily. If an automatic drain device is used, as shown in
Fig. 1, the automatic drain should be checked
periodically to determine if it is functioning properly.
When manually draining tanks, satisfactory draining is
only accomplished by leaving the drain cocks open after
all air has escaped and all drainage has stopped.
Reservoirs are tested against a 2068 kPa (300
psi) and treated on the inside with a rust preventive.
With the air brake system charged, use Leak
Detector Tester (SE-2326) to check for leakage on
outside surfaces of reservoirs. If any leakage is found,
replace the reservoir.
Inspect inside and outside surfaces for damage
or corrosion. A small flashlight is helpful when inspecting
the interior. If damage or corrosion is found that would
weaken the reservoir, replace the reservoir.
Moisture taken in with the air through the
compressor inlet valve collects in the reservoirs
and necessitates draining the reservoirs daily in
cold weather and every week
in warm weather by opening the drain cock
located on the bottom. Be sure to close the
drain cocks after all moisture has been removed.
Drain air reservoirs regularly as required. Local
conditions govern frequency. In dry climates, for
example, once a month may be sufficient, while in humid
areas it may be necessary to drain reservoirs daily.
When draining the air reservoirs, open the drain
cock and let the air bleed off. Be sure to leave the cock
open until all drainage stops.
Drain cocks have a brass body fitted with a
tapered brass key. The drain cock is open when the
handle is parallel to the body and closed when the
handle is at right angles to the body. Drain cocks are
installed in the bottom of each reservoir (Fig. 1) in the air
brake system to provide a convenient means of draining
Always open a drain cock by hand. Never strike
the handle with a hammer or any other instrument, as
the cock will be damaged and leakage will develop.
Fig. 1 Two Compartment Reservoir and Drain Cocks
Automatic Drain Valve
With air brake system charged, test with soap
suds for leakage past the key. Also check for
leakage through the body by coating the outside
of the drain cock with soap
CTS-4079 - CHAPTER II
PRINTED IN UNITED STATES OF AMERICA