Question A low air pressure warning signal should activate: Answers When tank pressure falls below 120 psi. When tank pressure falls below 60 psi. When tank pressure falls below 10 psi. Explanation In an air brake system, a low air pressure warning signal must come on if air pressure in the tanks falls below 60 psi. This warning signal may come in the form of a light, a buzzer, or a wig wag.