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Question:

Semi-trailers made before 1975 that are equipped with air brakes:

A Often do not have spring brakes.
Explaination

Newer trailers have spring brakes just like trucks and truck tractors. However, converter dollies and trailers built before 1975 are not required to have spring brakes. Those that do not have spring brakes have emergency brakes, which work from the air stored in the trailer air tank.