Page 49 - 2021-2022 Commercial Drivers Manual

SECTION 5 : AIR BRAKES

LOW AIR PRESSURE WARNING DEVICES

Figure 5.5

gloves and pull hard on each slack adjuster that friction area. Linings (friction material) must not ing brake valve should close (pop out) on other

you can reach. If a slack adjuster moves more be loose or soaked with oil or grease. They must combination and single vehicle types when the

than about one inch where the push rod attaches not be dangerously thin. Mechanical parts must air pressure falls to the manufacturer’s speci-
to it, it probably needs adjustment. Adjust it or be in place, not broken or missing. Check the air fication (20 – 45 psi). This will cause the spring

have it adjusted. Vehicles with too much brake hoses connected to the brake chambers to make brakes to come on.
slack can be very hard to stop. Out-of-adjust- sure they aren’t cut or worn due to rubbing.
ment brakes are the most common problem Check Rate of Air Pressure Buildup. When
found in roadside inspections. Be safe. Check 5.3.3 – Step 7 Final Air Brake Check the engine is at operating rpms, the pressure
the slack adjusters. should build from 85 to 100 psi within 45 sec-
All vehicles built since 1994 have automatic Do the following checks instead of the hy- onds in dual air systems. (If the vehicle has
slack adjustors. Even though automatic slack draulic brake check shown in Section 2, Step 7: larger than minimum air tanks, the buildup
adjustors adjust themselves during full brake Check Brake System. time can be longer and still be safe. Check the
applications, they must be checked. Test Low Pressure Warning Signal. Shut the manufacturer’s specifications.) In single air sys-
Automatic adjusters should not have to be engine off when you have enough air pressure tems (pre-1975), typical requirements are pres-
manually adjusted except when performing so that the low pressure warning signal is not sure buildup from 50 to 90 psi within 3 minutes

maintenance on the brakes and during installa- on. Turn the electrical power on and step on and with the engine at an idle speed of 600-900 rpms.
tion of the slack adjusters. In a vehicle equipped off the brake pedal to reduce air tank pressure. If air pressure does not build up fast enough,
with automatic adjusters, when the pushrod The low air pressure warning signal must come your pressure may drop too low during driving,
stroke exceeds the legal brake adjustment limit, on before the pressure drops to less than 60 requiring an emergency stop. Don’t drive until
it is an indication that a mechanical problem psi in the air tank (or tank with the lowest air you get the problem fixed.
exists in the adjuster itself, a problem with the pressure, in dual air systems). See Figure 5.5. Test Air Leakage Rate. With a fully-charged
related foundation brake components, or that If the warning signal doesn’t work, you could air system (typically 125 psi), turn off the en-
the adjuster was improperly installed. lose air pressure and you would not know it. gine, release the parking brake, and time the

The manual adjustment of an automatic ad- This could cause sudden emergency braking air pressure drop. The loss rate should be less

juster to bring a brake pushrod stroke within in a single-circuit air system. In dual systems than two psi in one minute for single vehicles

legal limits is generally masking a mechanical the stopping distance will be increased. Only and less than three psi in one minute for com-
problem and is not fixing it. Further, routine ad- limited braking can be done before the spring bination vehicles. With the air pressure built
justment of most automatic adjusters will likely brakes come on. up to governor cut off (120-140 psi), shut off the
result in premature wear of the adjuster itself. Check That Spring Brakes Come On Auto- engine, chock your wheels (if necessary), release
It is recommended that when brakes equipped the parking brake (all vehicles), and the trac-
with automatic adjusters are found to be out matically. Continue to fan off the air pressure tor protection valve (combination vehicle); and
of adjustment, the driver take the vehicle to a by stepping on and off the brake pedal to reduce fully apply the foot brake. Hold the foot brake
repair facility as soon as possible to have the tank pressure. The tractor protection valve and for one minute. After the initial pressure drop,
problem corrected. The manual adjustment of parking brake valve should close (pop out) on a if the air pressure falls more than three psi in
automatic slack adjusters is dangerous because tractor-trailer combination vehicle and the park- one minute for single vehicles (more than four
it may give the driver a false sense of security re-
garding the effectiveness of the braking system.
The manual adjustment of an automatic ad- TEST YOUR KNOWLEDGE
juster should only be used as a temporary mea-
sure to correct the adjustment in an emergency Subsection 5.2 and 5.3
situation as it is likely the brake will soon be back
out of adjustment since this procedure usually • What is a dual air brake system? • How can you check that the spring
does not fix the underlying adjustment problem. brakes come on automatically?
(Note: Automatic slack adjusters are made by • What are the slack adjusters?
different manufacturers and do not all operate • How can you check slack adjusters? • What are the maximum leakage rates?
the same. Therefore, the specific manufacturer’s
Service Manual should be consulted prior to • How can you test the low pressure
troubleshooting a brake adjustment problem.) warning signal?
Check Brake Drums (or Discs), Linings, These questions may be on your test. If you can’t answer them all, re-read subsections
and Hoses. Brake drums (or discs) must not 5.2 and 5.3.
have cracks longer than one half the width of the
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