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Old 12-14-2019, 12:56 PM   #1
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Slack Adjusters....

So I have a new to me National on a Freightliner XC chassis.... I have this long list of issues to INSPECT..


While changing the oil in the gen set for the first time in 8 years.... yep... that long... as the old filter was marked...

I had the wife touch the brake pedal and I watched the travel of the slacks...


Per DOT that should be something like 5/8" to 3/4"... I noticed that both the front went much further...
I hope most of you understand that foot pedal travel and stroke in the spring brake have NO MECHANICAL connection... and the spring brake has a LIMIT of about 1 3/4 to 2" unless its a long stroke can...

Of course my unit has self adjuster (BENDIX) but I was unhappy with the the adjustment and reset it....
What worries me is how many units have out of adjustment slacks so that you really CAN'T get full brake application PRESSURE.... because of the limited travel.... and you won't know this unless you have to dynamite them in an emergency.


Well you might know this if your unit rolls when the parking brake is applied.... the spring brake is stroked out (at it's limit) and can't apply full parking brake pressure.
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Old 12-14-2019, 01:06 PM   #2
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Glad you found that problem now, now when it was too late.

Don't know what the year of your coach is, but I wonder if the slack adjusters have ever been maintained/greased/tended to?

Were you able to get the brakes properly adjusted manually, keeping both sides pulling evenly?
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Old 12-14-2019, 01:34 PM   #3
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slacks

On trucks you check to make sure travel is less than an 1.5 inches,if not,replace the slackas the internal gears are stuck or stripped.I've never heard of the5/8 to 3/4 spec.When i do a manual adjustment,I try for about an inch.My 2cents,been a truck tech since 72.
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Old 12-14-2019, 02:11 PM   #4
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Originally Posted by ronster View Post
On trucks you check to make sure travel is less than an 1.5 inches,if not,replace the slackas the internal gears are stuck or stripped.I've never heard of the5/8 to 3/4 spec.When i do a manual adjustment,I try for about an inch.My 2cents,been a truck tech since 72.

This is the way I always understood it too.
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Old 12-14-2019, 02:38 PM   #5
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Here is how it actually works.

https://www.todaystrucking.com/brake...-make-me-stop/

Despite what you may think, the 2-inch stroke limit on a standard Type-30 brake chamber isn’t arbitrary. With a Type-30 chamber, 0.66 inches of pushrod travel is used to move the brake lining from its resting position to the point of contact with the drum. This is sometimes referred to as “free stroke.” The next 1.15 inches is called the power stroke. Here we have the foundation brake components twisting, bending, and stretching as the slack-adjuster arm exerts more force.

You can sees that it's .66" until point of contact, not actual brake application.

A critical point is that when the brakes are applied there is still slightly less then a 90° angle between the rod and slack adjuster. That's the point of the most mechanical advantage.Click image for larger version

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Old 12-14-2019, 03:23 PM   #6
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Even theough they are "automatic" I was taught to check them anyhow, just as we always did the older manual slack adjusters. They were usually in pretty good shape, but occasionally one would be out quite a bit. I still check mine.
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Old 12-14-2019, 07:00 PM   #7
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According to the DOT, If a slack adjuster is out of adjustment it needs to be fixed or replaced.

Just adjusting it is not safe, it will go out of adjustment quickly.
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Old 12-14-2019, 09:52 PM   #8
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Quote:
Originally Posted by twinboat View Post
According to the DOT, If a slack adjuster is out of adjustment it needs to be fixed or replaced.

Just adjusting it is not safe, it will go out of adjustment quickly.


Many go out of adjustment because the automatic adjuster is not properly lubricated when greasing the chassis, and sticks. Then a manual adjustment is required until you reach a repair shop. USDOT does not recommend a manual adjustment under normal conditions; if a manual is required to restore braking, a brake inspection is mandatory.
Twinboat;

The last paragraph of this document re-enforces what you stated about adjustment length .https://www.ccmpool.com/UploadedDocu...at%20FMCSA.pdf
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