Vacuum Brakes

All British first generation DMUs used the Gresham & Craven quick release twin-pipe vacuum brake system, with the exception of the railbuses which had air brakes.

Release of the brakes is entirely independent of the exhauster speed and is obtained through the medium of a 'release reservoir' which is exhausted while the car is running.

General arrangement - showing a power car and driving trailer

A Driver's brake valve B Passenger communication valve with operating lever C Duplex gauge D Feed valve E Release reservoir F Drivers safety device (DSD) G Brake cylinders H Hose and coupling for train pipe J Hose and coupling for exhauster pipe K Non-return valve L Single gauge (guards van) M Van valve	N Bend P Hose and clips Q Filter R Automatic Isolating valve S Exhauster T Dummy and carrier (train pipe) V Release valves W Dummy and carrier (exhauster pipe) X Hose and clips for cylinder Y Hose (underframe to bogie) Z Oil separator

Also of note is the direct admission valve beside the brake cylinders, and the Passenger Communication and Guards Emergency Valve. When operated, these valves admit air into the train-pipe, giving a full brake application with no time delay.

Operation of the brakes

Running The car is in motion and the exhauster is at maximum speed. The feed valve prevents the train-pipe vacuum from rising above 21 inches. To do this it does not admit air like an ordinary relief valve, but shuts down at 21 inches train-pipe vacuum, thereby isolating the exhauster from the rest of the system. The exhauster then creates up to 29 or 29 inches of vacuum in the release pipe and reservoir, giving storage capacity for subsequent brake releases. The driver's brake handle is in the off position.

A High-vacuum release chamber B Feed valve C Driver's brake valve D Vacuum brake cylinder E Exhauster F Automatic isolating valve

Lap The driver's brake handle is in the lap position. The train-pipe is isolated from the feed valve and release pipe. The rain-pipe is also isolated from the atmosphere. In this position a partial brake application can be held. The drivers handle can only be removed in this position.

Brake On Direct admission of air from atmosphere into the train-pipe to apply the brake. High vacuum via feed valve sealed off and thus preserved. Partial applications can be maintained by returning the handle to the lap position.

Brake Release The driver's brake valve now links the train-pipe with the release reservoir via the feed valve. Air from below the vacuum brake piston and from the train pipe flows rapidly through the feed valve into the reservoir which is of sufficient volume to absorb all the air in the system. Immediately 21 inches is reached in the train-pipe, the feed valve closes as before. The auto-isolating valve is open.

Brake Release The auto-isolating valve is closed and 19 inches of vacuum is maintained in the reservoir.

For a document on brake percentages for each type of vehicle, click here.

Some vehicles had this plaque fitted on the driver's desk: