Making a heavy service brake application while the parking brakes are applied.
This is often an error of truck driving school students.
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This adding together of the two forces can potentially cause damage to slack adjusters, s-cams, brake chambers, brake shoes, and brake drums (the foundation brake components).
Yet, this problem is now obsolete, as the majority of trucks are equipped with an anti-compounding valve.
This valve prevents the service brakes from being applied if the parking parking brakes are engaged.
Despite this advance in technology, a significant number of driving instructors are going to get “right torqued” if a students makes a heavy service brake application and the parking brakes are engaged!