The DiaCompe Hombre is a great upgrade for a bikes using a U-brake and can be run front or rear.

Universal – Fits Front or rear, double or single cable routing.
Low Profile U-brake with V-type brake shoes.
ESSENTIAL upgrade from the standard U-brakes that come on most bikes, pound-for-pound the best way to improve your brakes!
Cold forged aluminium arms.
Independent Dual tension springs.
Low-pro 2 coil stainless springs & alloy caps.
Complete with Hombre V-type pads.
Comes with green fiesta pack.
Mounting bolts, bridge, straddle cable & cable tidy included