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Old 11-27-2018, 10:42 PM   #1
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S cam rotation test, more then 120 dgrs. 147k miles

Hi,


My bus is at the garage for the provincial inspection in BC. They found that the rotation of the cam is outside of spec. From some googling it looks like it's this test:



Foundation Brake Analyzer Instructions


They say that it might be that the brake drum is worn down far enough to cause this. That would mean a very expensive repair since apparently the whole casting needs to be replaced.


The bus has 147k miles on it.


Is this an expected wear item around this mileage, and is this known as an expensive repair?


Tx
Bas
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Old 11-28-2018, 01:13 AM   #2
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As a rule of thumb the slack adjuster should not have to move more than an inch to apply. To keep that 1" travel, as the brake lining wears out and the brake drum gets ground down, the slack adjust is adjusted so that the stroke stays the same. The problem occurs when the slack adjuster has moved so far around the clock that it is very probable that in a panic stop the rollers will go right up and over the top of the 'S'-cam and you will end up with no braking.

What they are measuring is what the angle is of the slack adjuster from when it is all the way back and the rollers are at rest to how far the slack adjuster has to travel to get the brakes to apply 100%. Less than 120* you are good to go. More than 120* and you are out of service.

One of several remedies can be applied.

First, new thicker foundation brake lining can be installed that will meet the current size of the brake drum and allow you to get another brake job out of the set of drums. That is as long as the drums are still within spe'c.

Second, new foundation brake lining and new drums can be installed.

Lastly, new pins, bushings, and slack adjusters can be installed to reduce the amount of slop in the system. The more slop the more the slack adjusters have to be adjusted so that everything is tight and up to spe'c. Adjustment of the slack adjusters uses of some of the "clock hours" between 0* and 120* so the tighter the system is the longer it will be between brake jobs.


Depending upon what wheels and wheel ends you have and what needs fixing the cost could be as low as $200.00 per wheel. If could also easily exceed $1000.00 per wheel.
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Old 11-28-2018, 08:46 AM   #3
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Originally Posted by brijn View Post
Hi,


My bus is at the garage for the provincial inspection in BC. They found that the rotation of the cam is outside of spec. From some googling it looks like it's this test:



Foundation Brake Analyzer Instructions


They say that it might be that the brake drum is worn down far enough to cause this. That would mean a very expensive repair since apparently the whole casting needs to be replaced.


The bus has 147k miles on it.


Is this an expected wear item around this mileage, and is this known as an expensive repair?


Tx
Bas
My bus only has 64k miles on it and has had the brake drums replaced already.
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Old 11-28-2018, 09:02 AM   #4
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Cowlitz nailed it. Brakes are a wear item and should be inspected periodically. As far as mileage goes it really depends on how large the brakes are and how often you stop. In an otr semi you'll get significantly more miles then out of a school bus.

Good shoe kits come as per hub unit(2 per axle) and include all the necessary springs, rollers, pins/bushings and retainers. While it's apart you inspect the s cam for radial and axial play along with wear on the cam face. Drums are sold separately and are usually replaced as well for being worn beyond spec.
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Old 11-28-2018, 10:50 AM   #5
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My bus only has 64k miles on it and has had the brake drums replaced already.

Thank you for that info! Can you share how much the repair was roughly (parts and labour)?


Tx
Bas
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Old 11-28-2018, 10:53 AM   #6
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@cowlitzcoach


Thank you very much for that detailed answer! This came as a bit of a surprise, since I assumed school district would do these inspections regularly. But maybe just crossed that threshold and they have not used it very much over the last few years


Bas
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Old 11-28-2018, 10:54 AM   #7
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Originally Posted by Booyah45828 View Post
Cowlitz nailed it. Brakes are a wear item and should be inspected periodically. As far as mileage goes it really depends on how large the brakes are and how often you stop. In an otr semi you'll get significantly more miles then out of a school bus.

Good shoe kits come as per hub unit(2 per axle) and include all the necessary springs, rollers, pins/bushings and retainers. While it's apart you inspect the s cam for radial and axial play along with wear on the cam face. Drums are sold separately and are usually replaced as well for being worn beyond spec.

They are pulling it all apart today, so I will know more soon


TX!


Bas
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Old 11-28-2018, 11:57 AM   #8
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Thank you for that info! Can you share how much the repair was roughly (parts and labour)?


Tx
Bas
You'd have to ask the Douglasville, Ga bus yard that. Got this puppy fully loaded, with low miles. Paid $1625 out the door for the whole bus!
Brand new brakes feel real good!
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Old 11-28-2018, 12:22 PM   #9
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You'd have to ask the Douglasville, Ga bus yard that. Got this puppy fully loaded, with low miles. Paid $1625 out the door for the whole bus!
Brand new brakes feel real good!

Rubbing it in eh!


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