Booster providing both hydraulic feed-back and hydraulic feel- back in a two-stage operation in which we boost-brake a vehicle. Concentric, nesting, inner and outer control parts include, as the sliding outer part, a brake-pedal-connected input signal spool or sleeve for developing a desired signal pressure varying linearly along the length of, and therefore according to the sliding position of, the sleeve; the inner part constitutes a captive, telescoping metering spool with its ends in corresponding ends of the spool sleeve. At one end, the metering spool is given the desired signal as a first stage pressure to respond to; the spool's response constitutes the second stage, by metering an actual pressure and applying it at the other end of the spool sleeve and spool, whereby hydraulic feed-back comes on in the second stage at actual pressure to responsively control the other end of the metering spool, and whereby hydraulic feel-back in the second stage comes on at the same actual pressure to exert itself on the other end of the spool sleeve and therefore react against the brake pedal.
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