Physical description
Drawing of the air brake system on a Melbourne tram using a self lapping brake and relay valve - the charging and release positions.
89.1 - shows the relay valve in the exhaust position
89.2 - shows the valve in the applied position.
Has been drawn in ink on a piece of paper, and the coloured red or green as necessary to show the various positions and state of the valves, reservoir, brake cylinder and pipes.