A valve assembly includes a valve housing defining therein a fluid flow channel between a first end arranged to be connected to a source of pressurised fluid, and an outlet a frangible member disposed within and across the fluid flow channel, rupture means for rupturing the frangible member and biasing means for biasing the means for rupturing into contact with the frangible member to cause rupture thereof. The assembly also includes retaining means for retaining the rupture means out of contact with the frangible member to prevent rupture thereof and release means configured to release the retaining means to allow the biasing means to bias the rupture means into contact with the frangible member. The retaining means includes a bearing extending to engage the biasing means and hold it in a compressed state and the assembly includes also includes securing means for securing the bearing in the engaged state.