When pressing the brake pedal to start the car, it sounds like air rushing out, Only on the first press. Everything afterwards is normal and ABS engages just fine under max pressure (due to an idiot in a traffic circle…).

Normal or not normal?

 

Short answer: perfectly normal.

Long answer: the brake booster uses engine vacuum to provide brake boost ("power assist"). Once the engine is shut off, the booster holds a small reserve of vacuum pressure. When you press the brake pedal (with engine off/ ie. at startup) a portion of this reserve is being vented to atmosphere. That is the brief "whoosh" sound you hear. The pedal will get harder to push as more of this vacuum is released. Perfectly normal.

 

Excellent. Thanks for the explanation and putting my kind at ease!