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Booster
From WikiPedalia
The bosses of cantilever brakes and centerpull brakes can flex outward when the brake is applied hard. This contributes to a "spongy" feeling to the brake lever, and, in some cases, the extra flex may permit the brake lever to bottom out against the handlebar.
A "brake booster" is a metal bridge that connects the outer ends of the two cantilever bosses, holding them in position and greatly reducing their tendency to flex under load. A booster resembles an inverted "U", as it curves up and over the tire.
