Vacuum Brake

The automatic braking system where the brakes on each vehicle are actuated by 
the action of atmospheric pressure over a pre-formed vacuum. The brake pipe- is 
normally evacuated by a motor driven exhauster to create a vacuum and release 
the train brakes.
  
The vacuum developed in the brake pipe is measured in inches of mercury and is 
usually in the range of 21 to 25 inches for a fully charged system. As the 
degree of vacuum was by no means standardised in the UK, this caused some 
problems on joint services. For example, cases are recorded where Southern 
Railway engines could not release the brakes on Great Western Railway trains 
because the GWR vacuum was higher and the SR engines could not create 
sufficient vacuum to equalise throughout the train. They had to destroy the 
vacuum completely and start again - this causing delays.
  
The system has fewer valves than the equivalent air brake system but it has the 
disadvantage that response time and braking distances for a given weight of 
train are usually longer - over 50% in most cases.
  
In an attempt to speed up the propagation rate, later versions were fitted with 
accelerator valves on each vehicle. As soon as this valve detected a reduction 
in vacuum level, it admitted air locally into the brake pipe and brake 
cylinder, thereby speeding up the application. As soon as vacuum pressure was 
restored, the valve closed to prevent further air intake.
  
One big advantage of the vacuum brake is the ability to graduate release as 
well as application. The air brake triple valve was designed to allow a 
graduated application but, once set in the release position, it could not stop 
the release until the air pressure in the auxiliary reservoir was restored. 
Modern air braking systems are designed to overcome this and allow graduated 
release.
  
The vacuum brake is obsolete as far as railway braking is concerned but it is 
still used by those older equipped lines around the world which were based on 
British practice. For example, there are still EMUs operating in South Africa 
with vacuum brakes.