Simon,
Dry charged air is what was always used by the Army for Saracen, Saladin, and Leyland, we had small bottles for ease of use, which were charged by a special compressor with an air dryer. As far as I am aware, London Transport had always used air as well for Routemasters. Whilst working for the army, other equipment came in service, such as Scammell EKA, Terex loading shovles, etc, with nitrogen charged accumulators.