I see

@Graingy Ahh, my apologies for the confusion... To start off, I just want to say that this model is unrealistic (as I've said earlier), and it is indeed missing a one-way valve to prevent the air from going back (I honestly overlooked that crucial part).

Also regarding the question as to how the air propels it in the model, we can see that I used an air brake which acted as a "bottle cap" that opens as the toy moves forward and the air escapes. So yes, there's a "critical pressure" wherein at a certain point, it causes the bottle cap seal to break thus letting the air escape which serves as a thrust. Although this contradicts my point in my reply to your comment earlier, the bottle cap in this model is somewhat different from what I had in my mind when I wanted to create this. I made it that way as an excuse for my laziness to create an exhaust valve in a more accurate manner. So I'm really sorry for causing confusion hehe...

@Suomii I'm confused. So presumably it's a one-way pump, though the model is missing the valve needed.
How does the air propel it? Is there a critical pressure causing it to break and gradually release as thrust?

@Graingy Like in real life? To be honest, what I've made is not that realistic, although the concept is like this:

It uses a manual air pump to fill the bottle with compressed air (the nozzle should be covered to prevent leak and should be only uncovered when it's filled with enough compressed air), and the compressed air is what propels the car itself... 😅

Cool. But how would it work?