Both ike, spring keeps it shut and the vac/ boost line keeps it shut aswell, when you close the throttle, the vac sucks the piston out and releases it, obviously…
Get a tial bov!
Or if you don’t want to weld a flange, use an evo9 metal one, crushed they are good for 30 psi… You can get one for probably 30-50 on an Evo forum or eBay.