and continental European practice had long favoured compressed air brake systems, the leading pattern being a proprietary Westinghouse system. This has a number of advantages, including smaller brake cylinders (because higher air pressure could be used) and a somewhat more responsive braking effort. However, the system requires an air pump. On steam engines this was usually a reciprocating steam-driven compressor, which was quite bulky and much more complicated and maintenance-intensive than the vacuum ejector, which was compact and had no moving parts. The compressor's distinctive shape and the characteristic puffing sound when the brake is released (as the train pipe has to be recharged with air) make steam locomotives fitted with the Westinghouse brake unmistakable. Another disadvantage of the earlier air brake systems (although later overcome) was that it was impossible to make a partial release. The vacuum brake can very simply be partially released by restoring some (but not all) of the vacuum, without having to fully release the brakes. The original air brake systems, on the other hand, did not allow this, the only way of partially releasing the brake being to fully release it, then re-apply it to the desired setting. A corollary of this was that the standard vacuum brake system, as used between the 1860s and the 1940s, could not easily maintain a constant level of application. The driver could remove air from the train pipe using the ejector(s) or admit air using the brake valve but there was no way of setting the brake to a fixed level of vacuum between 'zero' (atmospheric pressure) and the maximum vacuum-generating capabilities of the ejector (21-25inHg, see above). The only way to do so was to carefully balance the setting of the brake valve and the small ejector, which was difficult to achieve in practice and was not even possible on some systems which combined both into a single control. This meant that braking occurred through a series of controlled applications and releases – perfectly adequate to safely bring a train to a halt but requiring constant management to maintain a speed on a downhill gradient. By contrast even the earliest Westinghouse air brake systems could be 'lapped' – the system would maintain the brakes at a constant level as set by the driver. Later vacuum brake systems as fitted to British Railways diesel and electric locomotives and multiple units in the 1950s used mechanically-driven exhausters or