What is the problem that the rear axle pressure of Isuzu EMU can't go up?

There are two kinds of pneumatic brakes, one is dead braking. When the pneumatic spring is removed, it will stretch against the brake arm and the brake will lock. That's why some cars don't leave until the air pressure goes up. The other is pressure braking, and air braking is locked. For safety, some cars like this have pressure protection. If the air pressure is insufficient, they will not turn off the engine. In order to turn off the engine, the car has brakes.