§393.40
Required Brake Systems
Question 1: May a system such as "driveline
brakes" be used as an emergency brake provided it complies with the
requirements of §393.52?
Guidance: Yes. CMVs which
were not subject to the emergency brake requirements of FMVSS Nos. 105 or 121 may use
"driveline brakes" provided those vehicles meet the requirements of §393.52.