A pneumatic lock assembly for securing a valve comprises a lock pin engaging a contact surface on a poppet assembly. The head of a poppet creates a sealing relationship with an end of the valve. A guide with a bore extends between the exterior and interior of the valve terminating at a sleeve. The sleeve slidably receives a stem extending from the poppet head. A spring biases a pin, slidably received within the pylon bore, into engagement with a contact surface on the valve stem for securing the valve head in a closed position. The pin may be pneumatically disengaged from the groove by forcing air from a pneumatic power source into a bore in the pin. A cap assembly at the exterior of the valve provides protection for the lock assembly and allows for manual disengagement of the pin from the groove.