Continuing to fan off air pressure during the Emergency Brake(s) Test, at what air pressure must the parking/emergency brake valve 'pop out' (close), and how must an air-operated trailer's protection valve compare in timing?

nhcdm-s.pdf Section 11M.1.2 - Air Brake Check, p. 11M-3; 49 CFR 383.111(b)(5)

This final test stage confirms the spring brakes will automatically apply before air pressure drops so low the brakes can't hold at all, and that the trailer locks up at or before the tractor so the rig doesn't jackknife or the trailer run free. Getting the pop-out order wrong is a direct crash risk during a real air-loss emergency.