Look closely at a brake caliper and you will see small rubber boots around the piston and guide pins. These dust boots are simple but important for caliper life. Here is what a caliper dust boot does.
What it is
A dust boot is a flexible rubber seal that covers the gap around the caliper piston (and around each guide pin). It keeps road grit, water, and salt out of the sliding surfaces while still letting the piston and pins move in and out.
Why it matters
If a dust boot tears, contamination gets into the piston bore or pin, causing corrosion that makes the caliper stick or seize - see caliper piston seal. A sticking caliper drags, overheats one wheel, and wears the pad unevenly; boots are the first line of defense against that.
Dust boot vs piston seal
The dust boot keeps dirt out; a separate square-cut piston seal inside the bore actually holds the brake fluid pressure and helps retract the piston - see how disc brakes work. Both are rubber and both age, but they do different jobs.
What to do
Inspect the boots at every brake job - a cracked or torn boot means the caliper needs a rebuild or replacement soon. Keep the slides clean and greased, and fit quality pads and hardware.