My bike had copper crush washers when I got it. Original flex line, so I'm guessing the factory used copper. Aluminium crush washers are available, but I don't know if they'd be up to the pressure.
FYI (I'm NOT recommending this for use in braking systems), copper crush washers can be re-used if they are in good condition and annealed prior to re-use.
FYI (I'm NOT recommending this for use in braking systems), copper crush washers can be re-used if they are in good condition and annealed prior to re-use.