The vacuum powered brake servo has a sliding control valve operated by an electro-magnet. A small permanent magnet (20) fitted to the outside of the sliding sleeve of the control valve is monitored by a Hall probe (21) fitted to the electro-magnet. The moving sleeve and the moving part of the control valve are fitted with silicon rubber damping to prevent loosening of the control system through impacts. The Hall probe is fitted to a slim support bracket (18) which is clipped over the outside of the electro-magnet (12). The wire connections to the Hall probe are fitted to the bracket and the whole fitting does not require any additional space.
