Adjusting Automatic Slack Adjusters

chasfm11 · 12-08-2008, 03:35 AM

On another forum, a discussion topic was started about the "need" to adjust automatic slack adjusters on an air brake system. For the record, the system on our Freightliner is a Meritor drum brake ABS air brake system. I believe that I'm doing all of the things that I should be regarding moisture in the system, periodic checks for leaks and proper lubrication of the slack adjusters. This thread is only intended to focus on the "adjustment" point.

One of the points made by other posters was that some State DOTs pull over OTRs and park them because of faulty brake adjustments. That seems to fly in the face of the purpose of automatic adjusters. There is an air brake manual, used as the basis for teaching drivers about air brake system that clearly says automatic slack adjusters should never be manually adjusted except for a qualified profession who is familiar with the adjustment procedure for that maker's slack adjuster. Other points made were about the slack adjusters having to be less than 90 degrees when the brakes are applied and there having to be an 1/8" gap between the shoe and the drum with the brake in the retracted system. One poster went so far as to say that the adjusters should be examined before every steep grade.

With all of the talk about "properly adjusted systems" and States forcing manual adjustments, I decided to ask the following questions here.
1. What is a "properly adjusted system?" How does one know that one (or more) of the automatic slack adjusters are not working properly?
2. The air brake manual states that the solution for automatic slack adjusters that are not properly adjusted is replacement of the slack adjuster. If this is true, why would anyone adjust them? It is my understanding that improper manual adjustment can cause a complete failure of the slack adjuster.

I'm always anxious about braking systems. Since no one else but me is working on our RV at the moment, I don't want to be missing something that will compromise our safety. On balance, I felt that the original thread that I read on this subject had some pontification without facts to back it up and that some posters were mixing manual slack adjusters with automatics in their discussions. That is why I decided to ask here instead of there.

Thanks,

Charlie

chasfm11 · 12-08-2008, 03:35 AM

On another forum, a discussion topic was started about the "need" to adjust automatic slack adjusters on an air brake system. For the record, the system on our Freightliner is a Meritor drum brake ABS air brake system. I believe that I'm doing all of the things that I should be regarding moisture in the system, periodic checks for leaks and proper lubrication of the slack adjusters. This thread is only intended to focus on the "adjustment" point.

One of the points made by other posters was that some State DOTs pull over OTRs and park them because of faulty brake adjustments. That seems to fly in the face of the purpose of automatic adjusters. There is an air brake manual, used as the basis for teaching drivers about air brake system that clearly says automatic slack adjusters should never be manually adjusted except for a qualified profession who is familiar with the adjustment procedure for that maker's slack adjuster. Other points made were about the slack adjusters having to be less than 90 degrees when the brakes are applied and there having to be an 1/8" gap between the shoe and the drum with the brake in the retracted system. One poster went so far as to say that the adjusters should be examined before every steep grade.

With all of the talk about "properly adjusted systems" and States forcing manual adjustments, I decided to ask the following questions here.
1. What is a "properly adjusted system?" How does one know that one (or more) of the automatic slack adjusters are not working properly?
2. The air brake manual states that the solution for automatic slack adjusters that are not properly adjusted is replacement of the slack adjuster. If this is true, why would anyone adjust them? It is my understanding that improper manual adjustment can cause a complete failure of the slack adjuster.

I'm always anxious about braking systems. Since no one else but me is working on our RV at the moment, I don't want to be missing something that will compromise our safety. On balance, I felt that the original thread that I read on this subject had some pontification without facts to back it up and that some posters were mixing manual slack adjusters with automatics in their discussions. That is why I decided to ask here instead of there.

Thanks,

Charlie

notslo · 12-08-2008, 07:27 AM

One of the things I do is make sure I regularly make a higher brake application. The general word is 60 lb application. My towed brake system requires 100 lb application to charge the break away so it also gets done then. If you want to check them manually here is a site that might be of use to you.
http://www.vosa.gov.uk/vosacor...to%20Maintenance.pdf

JRP · 12-08-2008, 07:52 AM

Charlie,

You can find your answers on pages 98-100 of this Meritor document. The adjustment proceedure they give is intended primarily for pre-setting a newly installed auto slack agjuster, and for confirming it is within tolerance. As they state on pg 99, you should never need to manually adjust a properly working unit.

http://www.meritorhvs.com/Meri...VS_Documents/mm1.pdf

"1. What is a "properly adjusted system?" How does one know that one (or more) of the automatic slack adjusters are not working properly?
2. The air brake manual states that the solution for automatic slack adjusters that are not properly adjusted is replacement of the slack adjuster. If this is true, why would anyone adjust them?"

1) since its based on maintaining the Mfg pre-detirmined gap between the brake lining and brake pad, you will notice a longer brake pedal movement, if its not automatically reducing that gap as needed. Alternately, you can measure the chamber travel as described by Meritor, and confirm its within tolerance. (this distance varies by type and size of brake chamber; and we have different chambers front/rear)
2) if the auto slack adjuster is not functioning properly, Meritor recommends a complete inspection of the entire brake system. it should not require manual adjustments.

you can see its a fairly simple mechanical device, when the brake chamber travelgap increases enough the push rod actuator engages the worm gear on release and causes the adjuster gear to rotate,decreasing the gap. As long as you keep that gear greased it should never be an issue.