What the walkaround owes you before step 7
The air brake checks are not all at the end. Section 5.3 of the manual fits extra items into the
earlier steps of the same seven-step method.
During the engine compartment check, if the air compressor is belt-driven, you check the condition
and tightness of that belt.
During the walkaround, you check the slack adjusters on S-cam brakes. Park on level ground, chock the
wheels, release the parking brakes so the adjusters can move, use gloves, and pull hard on each one
you can reach. If a slack adjuster moves more than about one inch where the push rod attaches, it
probably needs adjustment. Everything built since 1994 has automatic slack adjusters, and the manual
is blunt that they still have to be checked — out-of-adjustment brakes are the most common problem
found in roadside inspections. You also check brake drums or discs for cracks, and the manual gives
the limit: no crack longer than one half the width of the friction area. Linings must not be loose,
oil-soaked or dangerously thin, and the air hoses to the brake chambers must not be cut or rubbed
through.
Get those wrong and the seven tests below will still pass. That is the point of doing both.
1. Governor cut-in and cut-out
Run the engine at a fast idle and watch the air gauge. Pumping by the compressor should start at
about 100 psi and stop at about 125 psi, and the manual says to check the manufacturer's
specification because that is what governs. When the gauge stops rising, that is cut-out. Then idle
the engine and step on and off the brake to bleed pressure down; the pressure should start rising
again at cut-in. Elsewhere the same manual gives governor cut-out as usually around 120 to 140 psi.
What it proves: the compressor is being switched on and off at the right pressures. A governor that
does not work may not keep enough air for safe driving.
2. The leak tests
There are two, and they are different tests with different numbers.
Static, with the brakes released: fully charge the system, which the manual puts at typically 125
psi, shut the engine off, release the parking brake, and time the drop. The loss should be less than
two psi in one minute for a single vehicle, less than three psi for a combination.
Applied, with the brakes held on: build pressure to governor cut-out — 120 to 140 psi — shut the
engine off, chock the wheels if needed, release the parking brake and, on a combination, the tractor
protection valve, then fully apply the foot brake and hold it for one minute. The pressure must not
drop more than three psi in that minute on a single vehicle, or four psi on a combination. The
California edition spells out two more cases: six psi for a combination of three or more vehicles,
and three psi where the towed vehicles are not fitted with air brakes at all.
What it proves: that the system holds air both at rest and under load. Leaks that are invisible with
the brakes off show up the moment pressure is pushed into the chambers.
3. Low pressure warning signal
Shut the engine off while there is still enough pressure that the warning is not already on. Turn the
electrical power on, then step on and off the brake pedal to fan the pressure down. The low air
pressure warning signal must come on before the pressure drops below 55 psi — in a dual system, before
the tank with the lowest pressure drops below 55.
What it proves: that you would be told. Without a working warning, you could lose air and not know
it, which in a single-circuit system means sudden emergency braking and in a dual system means a
longer stop with only limited braking left before the spring brakes set.