Keep in mind that, most manufactures gas caps have a oneway breather valve by design so fule can not leak out the breather, so the test you are doing should be in reverse, you should be pulling a vacuum on the tank.

i have the lock cap ones how do i open ittttt 😭😭

No idea what others are talking about in the comments. These caps are supposed to let pressure out after a certain psi. Having some tank pressure is normal, excessive pressure is bad. Sigh, there's so much misinformation on the internet.

The cap is not supposed to let air out

Nothing like a good urm and arr to stretch a video.

Are you stoned?

These caps are examples of stupid engineering.