What sound indicates that the trailer brakes are functioning after air supply?

The hissing sound indicates that the trailer brakes are functioning after air supply because it is the sound of air being released as the brake system pressurizes and engages. When the air supply is established to the trailer brakes, the sound signifies that the air valves are working correctly and that the air pressure is building up in the brake system, which is essential for proper braking functionality.

The other sounds, such as a grinding noise, might suggest mechanical issues or brake wear, which does not indicate normal functioning of the air brake system. A clicking noise could relate to other mechanical parts but isn’t specifically associated with the trailer brake operation. Lastly, silence would suggest that there is no air movement or brake engagement occurring, which would be a signal of potential problems with the air supply or the braking system itself. In summary, the hissing sound is a normal indicator of functioning trailer brakes, as it represents the correct operation of the air brake system.