Since the introduction of hydraulic brakes on trucks in 1936, Chevrolet and GMC had been using the Huck brake system. On light trucks and cars it can easily be identified by the existence of a pair of shoe adjusting holes in each backing plate. This system requires each brake shoe to be individually adjusted to the adjacent drum face by turning a cogged wheel on each side of the wheel cylinder. Thus the system does not use a primary or secondary shoe as in more modern types. Front and rear shoes in each drum are the same.
In 1951, light trucks and cars began using the Bendix brake system. Medium and heavy duty GM trucks converted to Bendix brakes in 1953. This system has a single adjuster on each backing plate and equally moves each of the two shoes. Thus the need for shoes with different amount of lining because wear levels are not the same on the primary and secondary shoes.
1936-1950 Huck Brake (above)
1951-1972 Bendix Brake (above)