| 摘要 |
A vehicle braking system 10 comprising a brake actuator 23 having a spring brake chamber 23a and a spring wherein supply of fluid to the spring brake chamber 23a causes the brake actuator 23 to move against the spring force to release the brakes and release of fluid from the spring brake chamber 23a causes the spring to apply the brakes. The system including a supply line 12, and an emergency brake apply valve 20 which has a control port 20c which is connected to the supply line 12, the emergency brake apply valve 20 being configured automatically to connect the spring brake chamber 23a to a low pressure region when the pressure at the control port 20c falls below a predetermined level. The system further including an electrically operable emergency braking override valve 36, 36' which has a first port 36a, 36a' which is connected to the supply line 12 and a second port 36b, 36b' which is connected to the control port 20c of the emergency brake apply valve 20 and which, when supplied with sufficient electrical power, acts to prevent loss of fluid pressure at the control port 20c of the emergency brake apply valve 20. |
