Yes they do when not greased on a regular basis. They can seize up internally.
Where I live if the truck was manufactured with automatic slack adjusters you can NOT replace them with manual ones. Check with your local DOT you never know.
Yes there are slack adjusters on railroad cars. They work much like the automatic slack adjusters on large trucks, and do the same job
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No. You'll have two slack adjusters per axle, assuming it's using s-cam foundation brakes all around - disc brakes don't have slack adjusters.
The difference between manual and automatic slack adjusters is the way in which the brakes are adjusted. Automatic slack adjusters have sensors that do not let the brakes get too loose or too tight. Manual slack adjusters have to be adjusted by hand and can be set too tight in some cases.
half inch or less
Looking through FMCSR 393.47, the only requirement I see in there is that the effective length of a slack adjuster on each end of an axle must be the same.
You don't. They adjust automatically.
Step on the brake pedal.
Volvos have automatic slack adjusters, which are dangerous to manually adjust. If your brakes are so far out of whack you think you might have to screw with the slack adjusters, you really need a brake job. Usually the slack adjuster must be backed off before you can remount the brake drums. The do have to be adjusted manually to get at least some drag on the drums before you set them automatically.
Two per axle.
Slack adjusters are the linkage between the pushrod in the brake chamber and the S-cam between the brake shoes.