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Old 05-17-2010, 07:28 AM   #1
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Air Brake Automatic Slack Adjuster Adjustment

I was reading up on air brakes and automatic slack adjusters and I was shocked to find out that in normal braking the brakes will not automatically adjust. This was a online pdf file put out by Meritor that makes the brakes. The article stated that to adjust the automatic adjusters you should do the following:

"Ensure reservoir air pressure is above 90 lbs and make a FULL brake application and release. Repeat this process 6 times. These brake applications will be adequate to cause the automatic slack adjusters to adjust . A light brake application will not cause the adjustment"

A full brake application is pushing the pedal as far down as you can. When driving you never push the pedal that far so it never adjusts. I went through my Monaco book from cover to cover and it does not tell you anything about doing this. I was a little surprised.
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Old 05-17-2010, 11:24 AM   #2
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Good info Mike. DW sent me a link explaining some crazy ritual to do just that. It kept saying "fan the brakes". I assumed it meant push the pedal down and release several times not the brakes are hot

Much more simple. Will add that to the to do list when picking up stakes.
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Old 05-17-2010, 11:29 AM   #3
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I too will want to give this a try. One question for those in the know. When stopping using anything other then light pedal pressure, I get a clunk/thunk sound from the front brakes. Is this due to the slack adjusters being out of adjustment?
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Old 05-17-2010, 11:50 AM   #4
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Brian, that is hard to say. You can try doing the adjustment and see if it changes anything. Do you get the sound when you apply the pedal or when you release the pedal? Does it hapeen the same on both sides of the front. If it doesn't go away it might be worth getting it checked. Sometimes the automatic slack adjusters fail or stick. There is a procedure for checking the brakes. You have to have someone apply the brakes with 100 psi in the tanks and measure the length of the pushrod on the brakes. I don't have any data on what the length measurements should be.

HD4Mark. I don't think you have to do it everytime you pull stakes. Maybe once a month. It doesn't hurt to push the pedal to the floor a couple of times before you move out. When you do that you should also monitor the tank pressures. I believe the normal pressure drop when applying the brakes is 8 to 12 lbs but I don't believe our gauges are calibrated that well. On the older manual adjusted brakes it used to be that if they dropped more than 12 lbs then the brakes were out of adjustment. Not sure that applies to us but if you know your brakes are good and you note the normal pressure drop then if you see more than that on a later check may indicate that you have a problem.
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Old 05-17-2010, 12:01 PM   #5
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Quote:
When stopping using anything other then light pedal pressure, I get a clunk/thunk sound from the front brakes. Is this due to the slack adjusters being out of adjustment?
My Spartan chassis has done that since nearly new, particularly on the right front. When I asked that question at the last Cummins place where I had mine serviced a mechanic overheard the question and immediately responded that the slack adjusters were sticking, and that they need to be lubricated "frequently." That was part of what I had serviced.
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Old 05-17-2010, 12:17 PM   #6
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I read this somewhere a while back but it stated a full brake application with max air pressure would tighten the slack adjusters under normal conditions. But now I'll do the 6 full brake applications after reading your post just to be sure. Excellent info thank you.
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Old 05-17-2010, 12:50 PM   #7
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Originally Posted by AFChap View Post
My Spartan chassis has done that since nearly new, particularly on the right front. When I asked that question at the last Cummins place where I had mine serviced a mechanic overheard the question and immediately responded that the slack adjusters were sticking, and that they need to be lubricated "frequently." That was part of what I had serviced.
Have not lubed since last fall so it is due (by time if not by mileage ). Guess I will do a lube before we head out to our next location.
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Old 05-17-2010, 12:54 PM   #8
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Bob, what I have read is that in normal driving you will never do a full brake application because you would put everyone through the windshield. If you stood on the brakes then that would be a full application. Just got to do it six times.

Brian, it would seem to me that the indications would be the same for having sticking slack adjusters to having not really adjusted them because either way you are going to have too much movement of the pushrods in the wheel's brake assembly.
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Old 05-17-2010, 01:08 PM   #9
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Here is something else ....you cannot have the parking brake set when you do the adjustment. So you have to have the motor running to keep the air pressure up with it in neutral with the parking brake off. It is probably best to chock the wheels so you have no movement.
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Old 05-17-2010, 04:45 PM   #10
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Sometimes that clunk/thunk sound from the brakes could be from the "S" cam rollers that need to be cleaned and lubricated. Do a search on this forum for "S" cam rollers and you can see how others have diagnosed and solved the problem.
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Old 05-22-2010, 11:33 AM   #11
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If you do the following Air Brake Test, or similar, Before Starting your trip (which you should), you will likely adjust the slack adjusters:

Brake System Check Procedure

(Note- Damage can occur to your service brakes if pedal is pumped hard while Park/Emergency brake is engaged.)

  • Be sure rig is on level ground and/or use chalks
  • Start the engine. Let the air pressure build up until the governor activates the air compressor cut-out release of air at about 120-130 psi.


Check the Air Compressor Cut-Out and Cut-In Function

  • Release the emergency/parking brake.
  • Press the brake pedal several times until you see the needle drop to around 85-90 psi.
At that point the governor should activate the air compressor cut-in and the pressure should build back to 100 psi in about 45 seconds.
Allow the pressure to build to cut-out at about 120-130 psi.

Check for Air Leaks
  • Shut off the engine, but leave the ignition in the "on" position.
Watch the gauge for one minute - there shouldn't be more than a 2 psi drop in pressure. (This checks for leaks from the air compressor to the tanks and the one-way check valves.)
  • Press the service brake (brake pedal) and hold it down for one minute. There should be no more than a 3 psi drop in pressure. (This checks for leaks with the system under pressure: the air lines, one-way check valves, and Diaphragms.)

Check Audio and Visual Low Air Pressure Alarms/ Adjust Auto Slack Adjusters
  • Turn the engine back on and let the compressor build pressure to the air compressor cut-out.
Turn the engine off but leave the ignition switch on.
Smoothly pump the service brake pedal until the low air warning light and buzzer activate. Devices must activate at not less than 60 psi.

Check the Emergency/Parking brake Function
  • Continue to pump the service brake pedal until the emergency brake pops out automatically. Application should occur between 20-45 psi.

  • Start the engine and allow the air compressor to reach cut-out level.
Remove any wheel chalks.
Shift into gear.
Apply your accelerator, but not over 1000 RPM.
Your Rig should not move.



End of Brake Check
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Old 05-22-2010, 05:51 PM   #12
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Dean, that is an excellent checklist. Not sure I would do that full test every morning. Maybe like you said before I start on a trip. I do not see where the slack adjusters are going to be adjusted because there is not enough full brake applications with full air tank pressure. Maybe you should add a step at the end.

Slack Adjustment

Ensure reservoir air pressure is above 90 lbs on both air tanks and make a FULL brake application and release (Hard application to the floor). Repeat this process 6 times. (These brake applications will be adequate to cause the automatic slack adjusters to adjust)

Note: A light brake application will not cause the adjustment
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Old 05-23-2010, 01:27 PM   #13
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"Check Audio and Visual Low Air Pressure Alarms/ Adjust Auto Slack Adjusters"

I understand that this is a recommended air brake test for the pre-trip inspection on a Commercial Drivers License.

It will take at least 6 applications of the pedal to get from 120 psi to 45 psi in the pretrip process.

I think the deed will be accomplished.

Try it and let us know what you experience.

It takes about 5 minutes to do the complete system test.

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Old 05-23-2010, 01:52 PM   #14
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According to what I read in the Meritor handbook the pressure has to be maintained above 90 lbs to adjust the slack adjusters.
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