After a vehicle's air system has completely drained, why can you not drive away the instant the engine starts, even though the compressor is already running?

1. The governor stays in the unloaded stage and sends no air to the tanks until 100 psi is reached
2. The spring parking brakes stay mechanically applied until the compressor rebuilds enough pressure (about 70 psi in the spring chambers, per the SGI manual) to compress the springs and release the brakes
3. The one-way check valves stay closed until the safety valve blows off at 150 psi
4. The automatic slack adjusters lock the brakes until pressure reaches 90 psi