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 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.