When driving a set of doubles, it is necessary to close which shut-off valve in the last trailer:

Question

Accepted Answer

A: Back valve.. Each shut-off valve should be in the open position, except the furthest one back on the last trailer being towed. The valves, or cut-out cocks, allow air to be turned off for the service and supply lines when other trailers are not in tow.

When driving a set of doubles, it is necessary to close which shut-off valve in the last trailer:

Illinois CDL Combination Vehicles Test Info