I usually pry off those black plastic sleeves, pop them in a tin and replace them with a chromed or stainless steel 10mm domed nut - I use the same nuts to replace the brake shoe nuts, they look beater than flat zinc plated hex nuts and if the thread length is right, use another one to replace the brake retention nut. While I'm about it, Mother's polish and a buffing wheel to make the arms, spacers and spring shiny then reassemble, greasing the pivots, all washers and threads and the tips of the spring and spring seats. Turns a tired old Weinmann into a really snappy brake!