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570.56—Vacuum brake assist unit and vacuum brake system.
The following requirements apply to vehicles with vacuum brake assist units and vacuum brake systems.
(a) Vacuum brake assist unit integrity.
The vacuum brake assist unit shall demonstrate integrity as indicated by a decrease in pedal height when the engine is started and a constant 50-pound force is maintained on the pedal.
(1) Inspection procedure.
Stop the engine and apply service brake several times to destroy vacuum in system. Depress the brake pedal with 50 pounds of force and while maintaining that force, start the engine. If the brake pedal does not move slightly under force when the engine starts, there is a malfunction in the power assist unit.
(b) Low-vacuum indicator.
If the vehicle has a low-vacuum indicator, the indicator activation level shall not be less than 8 inches of mercury.
(1) Inspection procedure.
Run the engine to evacuate the system fully. Shut off the engine and slowly reduce the vacuum in the system by moderate brake applications until the vehicle vacuum gauge reads 8 inches of mercury. Observe the functioning of the low-vacuum indicator.
(c) Vacuum brake system integrity.
(1)
The vacuum brake system shall demonstrate integrity by meeting the following requirements:
(i)
The vacuum brake system shall provide vacuum reserve to permit one service brake application with a brake pedal force of 50 pounds after the engine is turned off without actuating the low vacuum indicator.
(ii)
Trailer vacuum brakes shall operate in conjunction with the truck or truck tractor brake pedal.
(2) Inspection procedure.
(i)
Check the trailer vacuum system by coupling trailer(s) to truck or truck tractor and opening trailer shutoff valves. Start the engine and after allowing approximately 1 minute to build up the vacuum, apply and release the brake pedal. In the case of trailer brakes equipped with brake chamber rods, observe the chamber rod movement. Run the engine to re-establish maximum vacuum, then shut off the engine and apply the brakes with a 50-pound force on the brake pedal. Note the brake application and check for low-vacuum indicator activation.
(ii)
For a combination vehicle equipped with breakaway protection and no reservoir on the towing vehicle supply line, close the supply line shutoff valve and disconnect the supply line. Apply a 50-pound force to the brake pedal on the towing vehicle and release. Trailer brakes should remain in the applied position.
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