The section applicants fail most often. Pressure figures differ by state manual.
Most Air Brakes questions are the same federal material in every state. One question in the app is written for North Carolina alone, from the North Carolina Commercial Driver's License Manual (2005 CDL Testing System, Version July 2014).
The manual used in North Carolina sets the visible low air pressure warning at 60 psi, higher than the 55 psi in the AAMVA model manual. Older vehicles may use one half the governor cut-out pressure instead.
One example, answer shown. Practicing the rest happens in the app.
The governor 'cut-out' pressure is about 125 psi. When tank pressure falls to roughly 100 psi, the governor cuts back in and the compressor resumes pumping.