It's not really an emergency brake, just a parking brake.
The owners manual says to use the parking brake in case of brake failure I mean I would use it anyway even if the manual didn’t say so because it’s a whole separate braking system also DOT requires a functioning parking brake this section is for single engine vehicles under 10k pounds with hydraulic brakes (my Yukon xL) If your car slides down a hill it will be investigated and you will be liable if your parking brake doesn’t work. :
Hydraulic-braked vehicles which were not subject to the parking
brake requirements of FMVSS No. 571.105 (S5.2) must be equipped with a
parking brake system that meets the requirements of
paragraph (c)
Paragraph (C)
(1) Each singly driven
motor vehicle not subject to parking
brake requirements of FMVSS Nos. 105 or 121 at the time of manufacturer, and every combination of
motor vehiclesmust be equipped with a
parking brake system adequate to hold the vehicle or combination on any grade on which it is operated, under any condition of loading in which it is found on a public road (free of ice and snow).
(2) The
parking brake systemshall, at all times, be capable of being applied by either the
driver's muscular effort or by spring action. If other energy is used to apply the parking
brake, there must be an accumulation of that energy isolated from any common source and used exclusively for the operation of the parking
brake.
Exception: This paragraph shall not be applicable to air-applied, mechanically-held
parking brake systems which meet the parking
brake requirements of FMVSS No. 121 (S5.6).
(3) The
parking brake systemshall be held in the applied position by energy other than fluid pressure, air pressure, or electric energy. The
parking brake system shall not be capable of being released unless adequate energy is available to immediately reapply the parking
brake with the required effectiveness.
Edit : Effective Aug. 15, 2005-present