While testing the tractor protection valve, you charge the system, shut off the engine, and pump the brake pedal repeatedly to bleed down tank pressure. At what point should the trailer air supply control pop out (or move to "emergency")?

Question

Accepted Answer

When air pressure falls into the range specified by the manufacturer, usually 20 to 45 psi.

Answer

Immediately when the engine is shut off, before any brake applications are made.

Answer

Only after tank pressure has been bled all the way down to zero psi.

Answer

At exactly 60 psi, a fixed figure used on the CDL general knowledge test.