The air pressure of the steam pump of agricultural vehicles is up, but it is out of air when braking. What is the reason?

This situation should be that the brake valve controlled by the brake pedal leaks from the air pipe or the driving valve of the wheel, so the air pump can pump up the air pressure when the brake is not applied, but once the brake is applied, the high-pressure air flow will leak to the air pipe below. This situation is not difficult to detect. You can go to a general repair shop to check and troubleshoot.