[Jimbuna]

In RL, vessels have at least two storage tanks for compressed air.....the main storage and a secondary tank.

When compressed air is called upon to perform a task, the air is drawn from the main storage, which is compensated from the secondary as it is being used/drawn.

Only when the main storage is empty would the guage drop, it would then rise again as the compressor feeds the secondary which in turn replenishes the main tank.

That is how it worked in RL and is replicated in game. This is my understanding anyway.