Jagwire 5mm POP End Cap for Compressionless Brake Bottle of 10 Durable alloy construction resists the increased stresses of compressionless brake housing Reducer end fits brake calipers and frames that are not designed for normal end caps POP end caps are designed specifically for use with 5mm compressionless brake housing. Not all frames or brake calipers are designed to accept a standardized end cap, so the POP end cap act