Final answer:
Air brake equipped trailers made before 1975 often do not have spring brakes but can legally be operated on interstate highways if they meet current safety standards, which may include using a glad hand converter for compatibility with modern air brake systems.
Step-by-step explanation:
When discussing air brake equipped trailers made before 1975, the correct answer is that they often do not have spring brakes. Spring brakes are a type of parking brake used in modern air brake systems. Trailers manufactured before 1975 might not have this feature because it was not a standard at the time. If these trailers are still in use, they may require certain modifications or additional parts, such as a glad hand converter, to connect with modern tractor units that have different air coupling interfaces. However, the legality of operating such trailers on interstate highways is determined by their state of maintenance and compliance with current safety standards, not solely by their age. It's important that the air brakes are properly maintained; for instance, the air in the brake lines must be bled out to ensure the brakes work efficiently, as air can be compressible which affects performance.
To address a common misconception, air brake equipped trailers made before 1975 can legally be operated on interstate highways provided they meet the necessary safety regulations and standards of today. The operation is contingent on proper maintenance and updates to meet current legal requirements.