A trailer braking system

A trailer braking system including: a supply line 44, a spring brake (connected to lines 102), which is released by pneumatic pressure and applied by spring when there is a lack of pressure; a spring brake control valve 10, which controls the flow of fluid to the spring brake; an immobiliser 80 (e.g. valve 84) which controls fluid flow to the spring brake control valve 10; and an auxiliary control valve 200, having a system enabled configuration and a system disabled configuration. In a brake release position the immobiliser permits fluid flow to the spring brake control valve 10, whereas in the immobiliser's immobilise position fluid is not permitted to the spring brake control valve 10. In the system enabled configuration, the immobiliser 80 is operable to move between its two positions. In the system disabled configuration, the immobiliser 80 is automatically placed in the immobilise position and fluid is not permitted to the spring brake control valve 10. Also claimed is a method in which immobilisation is caused by an electronic control unit (ECU) sending a signal to the auxiliary control valve 200, and mobilisation is caused by the ECU sending a signal to an override valve 54.