Metal-to-metal joints on fluid lines cannot seal perfectly due to surface scratches. A copper crush washer solves this. Made of soft, pure copper, the washer sits between the fitting and component. As you torque the bolt, the copper deforms, molding into the shape of the metal surfaces to plug all micro-gaps. This creates a reliable, high-pressure seal on oil pan drain plugs and brake caliper banjo bolts.
Copper hardens under pressure (work-hardening). Once crushed, the washer loses its softness and will not deform a second time. If you reuse a crushed washer, it will leak fluid. Always replace the washer with a new one at every oil change or brake hose service.