1) To convert the forward linear motion of the push rod into rotary motion of the camshaft.
The slack adjuster works on exactly the same principle as a box end wrench on a nut.
2) To act as a lever, thus generating greater force;
3) To provide a means of reducing the clearance between the brake shoes and the brake drum.
Canada mandated that all new equipment manufactered after 1996 was to be fitted with automatic slack adjusters. Although most air brake students are taught how to adjust manual slack adjusters, it is unlikely that drivers in this day and age will encounter a manual slack adjuster.
Stores compressed air for braking and for other air operated accessories.
The air tanks are the first fail-safe in the air brake system.
In the unlikely event that the air compressor fell of the side of the engine, the air tanks (reservoirs) store enough air for approximately 8-12 full service brake applications - more than enough to bring the vehicle to a stop.