General. The brake system: Shall be designed and constructed so that a single failure anywhere in the brake system, except for mechanical parts of the wheel brake assemblies, the brake pedal, and the brake pedal attachment to the brake valves or the master cylinder, does not leave the vehicle without brakes; and May not allow the vehicle to be without operative brakes capable of stopping the vehicle when loaded to the manufacturer's rated gross vehicle weight at any legal road speed. Hydraulic. Buses using a vacuum in operation of the brake system shall be equipped with warning signals, readily audible and visible to the driver, which shall provide a continuous warning when the vacuum for braking is less than 8 inches of mercury. An illuminated gauge shall indicate to the driver the inches of mercury/vacuum available for operation of the brakes. Vacuum assist brake systems shall have a reservoir used exclusively for brakes and shall be adequate to ensure loss in vacuum at full stroke application of not more than 30 percent, with the engine not running. The brake system on gas-powered engines shall include suitable and convenient connections for the installation of a separate vacuum reservoir. The brake system dry reservoir shall be safeguarded by a check valve or equivalent device. In the event of failure or leakage in its connection to the source of compressed air or vacuum, the stored dry air or vacuum may not be depleted by leakage or failure. Buses using a hydraulic assist booster in the operation of the brake system shall be equipped with warning signals, readily audible and visible to the driver, that shall provide continuous warning in event of a loss of fluid from the primary source or loss of electric power in the back-up system. Brake lines, plus booster assist lines, shall be: Protected from excessive heat and vibration; Installed to prevent chafing, cutting, or stretching. The parking brake system shall be designed and assembled to meet the following requirements: The parking brake shall hold the vehicle stationary or to the limit of traction on braked wheels, on a 20 percent grade under any condition of legal loading on a surface free from snow, ice, or loose material; When applied, the parking brake shall remain in the applied position with the capability as set forth in this regulation, despite exhaustion of the source of energy used for application or leakage; The parking brake control mechanism: May be mounted on the dash or on the floor, Shall be readily available to the driver, and When in the applied position, may not protrude into the walkway; and If floor mounted, there shall be a minimum 30-inch walkway measured from the barrier to the parking brake mechanism. A drive-line type parking brake shall have a conveniently mounted control to apply and release. A manual release shall be provided in cases when hydraulic pressure cannot be built up to release the brake. A warning light shall be provided to alert the driver that the parking brake is or is not applied. Air Brakes. Vehicles having full air brake systems shall be equipped with: A safety valve to protect the air system against excessive air pressure build-up; An illuminated air gauge in the instrument panel to register the air system pressure, with an audible and visual low pressure indicator to warn the operator when air pressure falls below 60 pounds per square inch; and A warning light to alert the driver that the parking brake is applied. Vehicles having full air brake systems shall be equipped with a check valve located between the source of air supply and reservoir. Brake lines shall be: Protected from excessive heat and vibration; and Installed to prevent chafing, rubbing, or stretching. Minimum brake lining size shall be 16.5 by 6 inches rear and 15 by 3.5 inches front. A fail-safe emergency brake system shall have a spring actuated chamber attached to rear brakes and shall automatically apply rear brakes upon loss of air. A brake system shall be of sufficient capacity for at least three brake applications and release after an engine stall. A manual control shall be installed in the driver's compartment, located so as not in reach of pupils to be readily available to the driver for emergency stopping or parking. If a treadle type is supplied, the treadle shall be either rubber covered or of a nonslip design and mounted directly on a valve. Treadle combination valves, involving linkages between valve body and pedal, are not permitted. Air lines, wires, etc., are to be supported or enclosed within the driver's compartment to protect against abnormal wear or damage. Rear air chambers are to be mounted in front of the rear axle. The system shall be equipped with an automatic, electrically heated drain system, with either an automatic drain valve or air dryer. The control of the emergency stopping system shall be designed and constructed to prevent release of brakes by the driver unless energy is available for reapplication.