Which plastic can be partially broken by pressure easily?
If you need good control, put a spring-loaded or cantilever-loaded valve at the tip.  Rupture diaphragms typically have a needle point at the far end. The bubble deforms with pressure, contacting the point that then ruptures the diaphragm that is under tension.  It is much easier to control and reproduce deformation than it is to engineer catastrophic failure at maximum deformation.