A valve mechanism for controlling release of pressurized fluid includes a fluid chamber. A plunger is received in an aperture in the fluid chamber, and is movable between a fluid retention position in which fluid is substantially trapped in the fluid chamber and a fluid release position in which fluid can escape from the fluid chamber through passages in the plunger. The plunger is biased into the fluid retention position. A driver drives a plunger actuator, and the plunger actuator is guided into engagement with the plunger to cause the plunger and the plunger actuator to move in unison and move the plunger into the fluid release position during a first portion of a stroke of the driver towards the fluid chamber, and the plunger actuator is further guided to permit the plunger to return to the fluid retention position during a second portion of the stroke.