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 of the large ejector when running. The widely used Super-Dreadnought ejector combined a large ejector with two small ejectors in one. If needed, two of the smaller nozzles could produce more vacuum, yet were more efficient in their use of steam then a single larger nozzle.