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Question:

If a converter dolly have spring brakes, you release the brakes by

A doing both of the answers.
Explaination

To connect the converter dolly to the first semitrailer, release the dolly's brakes so it can be moved. You release the dolly's brakes by opening the air tank petcock (valve) or using the dolly's parking brake control if the dolly is equipped with spring brakes.