vacuum pumps which included regulator valves allowing the driver to set a desired vacuum in the train pipe which would then be maintained by the system admitted or exhausting air as required. In the UK, the Great Eastern Railway, the North Eastern Railway, the London, Chatham and Dover Railway, the London Brighton and South Coast Railway and the Caledonian Railway adopted the Westinghouse compressed air system. It was also standard on the Isle of Wight rail system. This led to compatibility problems in exchanging traffic with other lines. It was possible to provide through pipes for the braking system not fitted to any particular vehicle so that it could run in a train using the "other" system, allowing through control of the fitted vehicles behind it, but with no braking effort of its own; or to fit vehicles with both braking systems. The Big Four companies formed in 1923 all chose to adopt the vacuum brake as the new standard for most rolling stock, with the same 21 InHg operating vacuum with the continuing exception of the GWR (and many electric multiple units introduced in this period used variations on the automatic air brake). A large proportion of the inherited air-braked stock had been retired or converted to vacuum operation by the outbreak of the Second World War and with the formation of British Railways in 1948 the 21 InHg vacuum brake system became the new standard. However, air-braked steam-hauled stock remained in service on the former Great Eastern Railway suburban lines from London Liverpool Street until the end of steam on the GE in 1962. Producing the vacuum Ejectors Vacuum brakes were initially favoured over air brakes because of the ease of producing the vacuum. A vacuum ejector was a simpler and more reliable device, compared to the reciprocating pump. Typically two ejectors are fitted, large and small. The large ejector is used to 'blow off' the brakes, by producing the vacuum, and is then shut off. The small ejector is left running continuously in order to maintain it. The Gresham & Craven 'Dreadnought' ejector was a combination ejector, with both large and small ejectors within the same body. The vacuum produced depended on the total number of vehicles in the train and the sum of their various minor leaks. A train of stock due for maintenance could make it difficult to maintain a vacuum, even requiring intermittent use