Air Brake convey (push pole travel) have to be checked as part of the pre–trip wait brake inspection (Section 9).

You are watching: How to check slack adjusters on air brakes

On vehicles equipped v hydraulic brakes that is feasible to pump the brake pedal to compensate for brakes that are out that adjustment. This is not possible with a vehicle equipped v an wait brake mechanism with hand-operated slack adjusters. As soon as the brakes are adjusted by slack adjusters, the shoes are moved toward the drum. This bring them as close to the north as feasible minimizing the lot of free travel once the brakes room applied. It likewise reduces the volume that air supplied to apply the brakes.

Your agency may have a maintain crew to keep vehicles safe running. But one human being alone is eventually responsible to ensure that the brakes room operating properly before the auto is moved. That human is the driver.


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Brake convey – It’s Critical

The most important part of any braking mechanism is the structure brakes. If these space not in correct working order and properly adjusted, the finest designed wait brake system won’t have the ability to safely prevent the vehicle.

Unfortunately, brake convey is frequently neglected. The reality is that brake failures and also runaways that an outcome in crashes are practically never brought about by an air device failure, but by the absence of program brake maintenance, or by the driver failing to check brakes ~ above a daily basis.

Daily pre–and post–trip inspections space mandatory. The driver might be hosted responsible if the brakes space incorrectly readjusted or not functioning properly.


Brake convey – It’s the Law

Federal and provincial laws require the manual and also automatic slack adjusters be checked daily, during the pre–trip inspection. It is up to you, the skilled driver, come ensure your vehicle has safe, correctly adjusted brakes. You the driver, can be fined if the brakes ~ above the car you room driving are found to be the end of adjustment.

You’re also required by regulation to check your brakes prior to driving under steep grades that are posted with regulatory signs.

You can’t constantly crawl under your car to measure up air brake room pushrod travel, but there space devices accessible to aid you visually check for brake adjustment.


S–Cam Brake–Release


S-Cam Brake–Applied



The most common kind of waiting brake chamber supplied on trucks and also trailers is the type 30. This chambers have a maximum obtainable stroke of 2M inches. A long stroke type 30 is designated form 30LS, and also has a maximum obtainable stroke of three inches.



In this diagram, the brakes have been applied and also the brake linings have contacted the brake drum. The brake chamber has actually stroked less than M of its maximum stroke, indicating the the brake is in exactly adjustment.

Each brake on an axle need to be changed to have a comparable amount of pushrod travel—that is, the pushrod travel of the left brake must be similar to the pushrod travel of the ideal brake ~ above the axle.


Checking and also Adjusting S–Cam Brakes–Manual Slack Adjusters


S–Cam Brake–Incorrectly readjusted and warm Brake Drum



This brake chamber pushrod has extreme travel, yet may work-related as long as the brake north is cool. But it won’t continue to be cool because that long!

Note the the brake chamber has actually stroked much more than ½ the its maximum, which way that this waiting brake chamber pushrod has too much travel and also the brakes are in need of adjustment.

When the brake drum is cool and with common brake application pressure, the brake will certainly seem to be effective, so that is simple to be lulled right into a false sense of security.

Air brake equipped vehicles rarely “lose” your brakes. Rather, brake failure occurs because the driver has actually failed come check and also correctly adjust the brakes.



Cast steel brake drums broaden when heated, leading to the air room to hit further and further together the temperature rises.

If an unforeseen event required the driver to do a sudden stop, the brake chamber could bottom out, and braking power would certainly be considerably reduced.

On long downgrades, the expansion of hot brake drums can reason a total loss of braking and result in a runaway.

Even incorrectly adjusted brakes might seem to work-related when brakes space lightly applied.

This could an outcome in brake fade under center to hefty braking, or ~ above a downhill.


S–Cam Brake–Incorrectly readjusted and Cold Brake Drum

Drivers seldom have actually the high-end of having actually the use of a pit and also hydraulic jacks so the brake adjustment have the right to be done through wheels turn off the ground. However, brakes deserve to still be adjusted very accurately with the wheels on the ground using among the complying with methods. Be sure that the auto is for sure parked v the wheels blocked. The air device should be at complete pressure and the feather parking brakes released. Revolve the engine off so the you have the right to listen for air leaks. There room two techniques of checking for correct adjustment, but the dimensions that show the require for adjustment room different. Pry method of free Stroke Measurement


You will need:

A wrenchA mite to measure up pushrod travelSturdy clothing suitable for crawling under the vehicleA flashlight

You should additionally wear eye protection and a bump cap. Come begin, do sure system air press is at least 90–100 psi and also all parking brakes space released.

Make a note on the pushrod wherein it exit the brake chamber.Note: it is sometimes difficult to note the pushrod in ~ this location. Alternatively, mark the facility of the clevis pen on the slack adjuster arm.Pull the pushrod the end from the brake chamber, utilizing a device for leverage.Measure the distance between the chalk mark and the challenge of the brake room ½ come ¾ the an inch of cost-free stroke is a great range because that a form 30 traditional stroke brake chamber. If this street is an ext than ¾ of an inch, the brakes require adjustment.

Applied stroke Method(service brake application)

The used stroke (brake application) an approach requires 2 people—one to use the brakes and also one to measure up travel.

If you have actually a an equipment to apply and also hold the organization brakes on, you deserve to use the brake application method without needing an additional person.

Checking the pushrod hit by applying the brakes.

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Brake adjustment Indicators

New waiting brake room pushrods have a noting (usually red) to indicate when brake adjustment should be excellent immediately. If the pushrod take trip becomes excessive, the marking will show.

Don’t wait until the red noting is exposed before adjusting the brakes.

If the red marking on the pushrod (at the room face) is visible, it indicates that the brake is dangerously the end of adjustment and it needs instant attention.

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The used stroke technique is the preferred technique and is also a technique recommended by commercial fleet maintain supervisors.

With the brakes released, make a mark where the pushrod exit the brake chamber.With the engine off, make a series of brake applications to alleviate the reservoir press to in between 90 come 100 psi.Apply and also hold a full brake applications (90 come 100 psi).Measure the distance in between the mark and the face of the brake chamber. The difference in between measurements is dubbed the chamber used stroke.

While checking air brakes look for other brake ingredient wear and excessive beat including:

S–cam bushing wearS–cam spline wearSlack adjuster beat (worn gear)Clevis pin wear

Brake Adjustment–Manual Slack Adjuster


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With a typical kind 30 conventional stroke brake chamber, friend must adjust the brakes if pushrod travel is:

more than ¾ of an inch utilizing the pry an approach of totally free stroke measurement, ormore than 1-½ inches making use of the brake application—applied punch measurement method.

Notethat these room maximum measurements. You should adjust the brakes if her measurements strategy these limits.

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Most hand-operated slack adjusters have actually a spring–loaded locking sleeve that need to be pressed in and held so the adjusting bolt have the right to be turned.

Depending upon the orientation that the slack adjuster, the correct direction to rotate the adjusting bolt may be clockwise or counterclockwise. Clock to ensure that you space tightening and not loosening the slack adjustment.

With a wrench that the appropriate size, normally a 9/!^ inch, boring the locking sleeve and also turn the adjusting bolt if watching the end of the camshaft. The camshaft will rotate slightly together the bolt is being turned. If you are turning in the ideal direction, the electronic came will rotate in the same direction the it would if the brakes to be being applied, as shown by the arrow on the cam end in the diagram below.

The push rod and also the slack adjuster arm should never relocate away from the brake chamber while turning the adjusting bolt.

If when turning the adjusting bolt, the slack adjuster is pulling the pushrod the end of the chamber, stop. The adjusting bolt is being turned in the not correct direction.

Once the appropriate direction is established, continue transforming until solid resistance is met. This indicates that the brake linings have actually contacted the brake drum.

If the brake has no dust shields, or if you have the right to see the brake shoes and linings with an inspection slot, you have the right to visually verify that the linings have actually contacted the drum.

Backing turn off the adjusting bolt about1⁄3of a turn should establish correct to run clearance in between the lining and also drum. Be certain the locking sleeve re–engages the bolt so that the adjustment will not back off.

It is common, particularly on tandem axle units, for the adjusting bolts top top one axle to readjust in one direction, while the other axle calls for an the opposite turn. It is not uncommon to discover that the brakes on one or the other axle have been mistakenly backed off, creating a serious safety hazard.

After adjusting, verify that there is adequate clearance by again pulling or prying on the slack adjuster arm, and re–measuring the complimentary stroke, remember the ½ customs is ideal.


Automatic slack adjusters room entirely various than the manual types.

Once effectively installed, automatic slack adjusters must not need hands-on adjustment. If an automatic slack adjuster is uncovered to stroke beyond the preferably allowed, this usually shows that there space brake difficulties that need to be repaired by qualified brake business personnel.

Automatic slack adjusters still have to be routinely checked come ensure the correct adjustment is being maintained.

Pushrod strokes v automatic slack adjusters space usually slightly longer than with well–adjusted hands-on slack adjusters. If a hands-on adjustment may temporarily shorten the stroke, the automatically slack will soon re–set come its design stroke.

Manufacturers recommend that automatic slack adjusters be checked by a mechanic at every chassis lubrication interval, or indigenous 40,000 come 80,000 km.

However, the national Safety password of Canada, and also federal and also state legislations in the U.S. Require a daily examine of brake adjustment as part of a pre-trip inspection. Also, in details mountainous locations of north America, van are required to stop and check brakes prior to proceeding down lengthy grades. The check is compelled regardless of whether hands-on or automatic slack adjusters are used.


Prior to manually checking slack adjuster stroke, for sure tractor spring parking brake control valve and trailer supply valve are open. For sure reservoir air pressure is above 90 psi and also make a complete brake application and also release. Repeat this procedure 6 times—these brake applications will certainly be enough to reason an automatically slack adjuster to adjust. A light brake application will certainly not cause the adjustment.

If after making 6 complete brake applications through reservoir pressure above 90 psi, a pushrod hit is excessive, the automatic slack adjuster has either failed, been mistakenly installed, or over there is a trouble within the foundation brake. Hands-on adjustment currently is only a stop–gap measure up so the car can be pushed to a company depot for inspection and feasible replacement.

Most manufacturers of automatically slack adjusters specify that pushrod stroke be confirm by making a 90 come 100 psi application. If you have no application push gauge, revolve the engine off and also pump the reservoir pressure down to in between 90 and 100 psi–then make a full application.

Automatic slack adjusters should only be changed or repaired by a default mechanic except in one emergency. Follow the manufacturer’s indict in transferring our any kind of adjustment.


Emergency manual Brake Adjustment—Automatic Slack Adjusters

THESE are EMERGENCY actions ONLY. A repair OR REPLACEMENT should BE made AS soon AS POSSIBLE.


Be certain that the vehicle is safe parked with the wheels blocked. The air device should it is in at full pressure and also the feather parking brakes released.

With a typical form 30 typical stroke brake chamber, friend must readjust the brakes if pushrod measurements are:

more 보다 ¾ of one inch utilizing the pry technique of cost-free stroke measurement, ormore than two inches using the brake application—applied punch method.

Four species of automatically slack adjusters

Note that these are maximum measurements. You should adjust the brakes if your measurements strategy these limits.

If you do an emergency mediate of an automatic slack adjuster, be sure to record it on your daily post–trip inspection report.

It is strong recommended that you obtain the company data publications for the do of automatic slack adjusters that you space using. They contain all the details needed to store them operating properly, consisting of lubrication requirements, early setup, inspection and testing. Castle are accessible where truck components are sold.


Slack Adjusters with Square Adjusting Bolts

If the slack adjuster has a square adjusting bolt located at the bottom end of the body, execute not effort adjusting until a spring–loaded pawl that meshes with inner teeth is disengaged. This units have actually a ¾ inch hexagonal (hex) cap situated on the slack adjuster body. Several of these hex caps room equipped through a round “button” that have the right to be pried increase approximately1⁄32of an inch and held, using a screwdriver. On systems not equipped through the button, the hex cap, spring and also pawl need to be removed. Through the spring and pawl disengaged, an adjustment can be made.

These brakes must be set up v a counter–clockwise turn until the lining contacts the drum. Turning the adjusting bolt ½ a turn clockwise will reclaim running clearance. Release the switch or re–install the spring and also pawl if they were removed. This kind of slack adjuster has a square adjusting bolt.


Wedge Brake Adjustment

Manual adjusters:

Hoist or jack wheels turn off the ground.Remove dust cover indigenous adjusting slot at two places on every brake. On twin chamber devices the adjusting slots room in the backing plate below the front chamber and above the behind chamber. If star wheel adjusting bolts are not uncovered at this positions, the brake has actually been assembled ~ above the wrong next of the vehicle. The single chamber units have adjusting slot in the backing bowl on every side the the chamber.Adjusting bolts have right–hand threads. Turning the wheel through hand, use an adjusting spoon to rotate the star wheel till the shoe creates a heavy drag ~ above the drum. Then earlier off the bolt to a very light traction on the drum.Repeat because that the various other shoe on the brake assembly.Replace the dust consist of in the adjusting slots.Repeat the above steps ~ above the various other brake assemblies.

There is no trusted substitute for physically checking brake adjustment.


After a Brake Adjustment

At the very first stop after adjusting the brakes, inspect each brake north or rotor because that temperature. An extra hot brake drum shows that you have actually over adjusted it.

Only when all brakes are properly readjusted will the system have the ability to absorb the required amount of power to bring the loaded auto to a safe avoid under too much conditions. Remember the poorly changed brakes may show up to be functioning at slow-moving speeds on irradiate air application. The only means to be certain your car brakes space properly changed is come physically check the press rod travel at each wheel.


In-Service Checks

In some areas signs room posted in advancement of steep or long downgrades. This signs indicate that the driver need to stop the auto in the pullout area and also inspect the braking system prior to proceeding. Examine that:

the compressor is maintaining complete reservoir wait pressure;push rod take trip is within restrictions on every chambers;there are no wait leaks;the glad hands and the lines space secure;the drums, bearings and tires space not overheating;the trailer supply valve is operation properly.

This must be offered as one in–service examine only and also not mistaken as a everyday pre–trip waiting brake inspection.

The driver should be conscious of the problem of the vehicle’s braking mechanism at every times. This have the right to be excellent by:

watching the push gauges;hearing the warning signals; andfeeling the braking solution of the vehicle.

By this an approach of observation, the driver have to be may be to an alert any defects developing in the braking system and also be conscious that service or convey is required.


Maintenance and also Servicing that the waiting Brake System

Ensuring that the vehicle is in appropriate condition to drive is the obligation of anyone involved.

Preventive maintenance checks space designed to stop mechanical failures the could result from neglect. This checks reduced repair prices by detecting minor mechanical defects which would certainly otherwise construct into major breakdowns.

Preventive maintenance may be assigned to a maintenance crew or come the driver. However the driver must ensure the the braking mechanism is in safe operation condition prior to the vehicle moves.

Click listed below for an ext Information


Air Brake adjustment Myths

There is some misinformation about air brakes the you may hear. Many of this is not critical, but the complying with three myths could be dangerous, if believed.

Myth #1: wait Brake adjustment deserve to be confirm from the cab by do a complete brake application and checking for an initial pressure drop of in between 8–12 psi. The presumption is that as the brake chambers hit further and further, that much more air volume will certainly be required, and this should present up top top the reservoir gauges.

Fact #1: modern trucks have actually very large air reservoirs, and also even if all the brakes had actually excess pushrod travel, the push drop would not with the 8–12 psi range. Also, most truck reservoir gauges perform not have actually markings fine sufficient to accurately estimate such pressure changes.

Myth #2:With the brakes applied, a 90-degree angle between the center of the slack adjuster arm and also the chamber pushrod is a good indication the the brake is in exactly adjustment.

Fact #2: The 90-degree edge is much more dependent top top the size of the room pushrod 보다 on brake adjustment. Also, to protect against interference in between the slack adjuster and also suspension parts, part manufacturers will differ the angle as much as plus or minus 10 degrees.

Myth #3:A clockwise revolve of the adjusting bolt top top a hand-operated slack adjuster will set up the brakes.

Fact #3: depending on the orientation the the slack adjuster ~ above the brake assembly, the exactly direction to collection up the brakes may be clockwise, or counter-clockwise. Top top vehicles with tandem axles, that is usual to find that one axle sets up with a clockwise turn, when the other requires a reverse direction.


Section Summary

Why is it so an essential to check slack adjustment?Who is ultimately responsible for the brakes on a vehicle?Who will be fined for operating a automobile with brakes improperly adjusted?What room the risks of operation a auto where the pushrod travel is barely within tolerance when the brake drums room cold?What items must you lug to measure and readjust slack adjusters?What is the very first thing you must do once preparing to measure slack adjustment?How lot pressure need to you apply to the foot valve as soon as measuring slack adjustment making use of the used stroke method?How perform you understand you are turning the adjusting bolt in the exactly direction as soon as adjusting a slack adjuster?What is an benefit of automatic slack adjusters?

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