JahLaza The spare spring in the box is the preinfusion spring, removed to disable the chamber. It’s designed to open around 4.5 bar when in it’s proper location. In the lower vent spring location withe the extra squeeze to fit it might get up to 6 bar?
Ideally you need a vent spring good to 12 bar. They are very cheap, but not many sites just sell the spring.
Of course there is probably nothing wrong with your old spring, would love to see a photo of the top and bottom cam follower pins.
How old is the machine?
P.S. The cam is harder than the top and bottom follower pins.