In the previous diagram, the compressor has raised the main reservoir pressure to maximum.

In the next diagram, the driver has opened the trailer supply valve (24) to allow the main reservoir air pressure to be directed through the tractor protection valve (23) to the trailer. The air pressure passes through the relay emergency valve (25) to the reservoir on the trailer. Pressure will build up in the trailer reservoir to the same pressure as the main reservoirs on the tractor. This is known as ‘charging’ the trailer system. The trailer supply valve will remain in the open position when the pressure has built up to between 20 and 45 P.S.I., depending on the make.