An anti-skid brake control system for motor vehicles including a housing, a power piston movable therein and dividing the interior of the housing into two chambers one on each side of the power piston, a conduit member connecting one of the chambers to an engine intake manifold, and a solenoid valve for controlling a pressure differential between the chambers in response to an input signal. The housing has an internally projecting guiding member on which is free to slidingly reciprocate a blind bore of the power piston having a plurality of axial grooves in its internal surface for storage of lubricant grease and communication of the blind bore with the interior of the housing.
A brake pressure control valve assembly is proposed which balances the input brake pressure with the output pressure and with a pressure difference between the atmospheric pressure and a controlled vacuum pressure proportional to the deceleration. Only a small part is always under the vacuum pressure and the regulating chamber, in which there is an inertia body, is under the atmospheric pressure while not braking and under the controlled vacuum pressure while braking. Most of the chambers are not under the vacuum pressure but under the atmospheric pressure.
