Check Rate of Air Pressure Buildup. When
the engine is at operating rpms, the
pressure should build from 85 to 100 psi within
45 seconds in dual air systems. (If the vehicle
has larger than minimum air tanks, the
buildup time can be longer and still be safe.
Check the manufacturer's specifications.)
In single air systems (pre-1975), typical
requirements are pressure buildup from 50 to
90 psi within 3 minutes with the engine at an
idle speed of 600-900 rpms.
If air pressure does not build up fast enough,
your pressure may drop too low during driving,
requiring an emergency stop. Don't drive until
you get the problem fixed.
Test Air Leakage Rate. With a fully-charged
air system (typically 125 psi), turn off the
engine, release the parking brake, and time
the air pressure drop. The loss rate should
be less than two psi in one minute for single
vehicles and less than three psi in one minute
for combination vehicles. Then apply 90 psi
or more with the brake pedal. After the initial
pressure drop, if the air pressure falls more
than three psi in one minute for single vehicles
(more than four psi for combination vehicles),
the air loss rate is too much. Check for air
leaks and fix before driving the vehicle.
Otherwise, you could lose your brakes
Section 5 AIR BRAKES
New York State Commercial Driver?s Manual
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