FERRULE

When you tighten the nut on a compression fitting, the ferrule deforms slightly and bites into the tube, holding it securely while blocking fluid or air from escaping. On heavy-duty trucks and trailers, this type of ferrule is common on air brake lines, fuel lines, and other low- to medium-pressure plumbing. Unless you find obvious wear, cracking, or a deep groove, ferrules are normally replaced whenever the fitting is taken apart.