Anti-skid apparatus for automotive vehicle

An anti-skid apparatus for installation in a vehicle braking system including a braking circuit connecting a master cylinder to a wheel brake cylinder, comprising:

• a bypass circuit connected in parallel with an intermediate portion of said braking circuit;
• a fluid reservoir connected to said bypass circuit to store an amount of hydraulic fluid;
• a hydraulic power pressure source disposed within said bypass circuit and connected to said fluid reservoir to produce a hydraulic power pressure higher than a braking pressure applied to said wheel brake cylinder from said master cylinder;
• first switchover valve means disposed within said braking circuit and said bypass circuit to be activated when a road wheel tends to be locked in braking operation, said first switchover valve means being arranged to permit the flow of fluid under pressure supplied to said wheel brake cylinder through said braking circuit in its deactivated condition and to interrupt said braking circuit and connect said wheel brake cylinder to said fluid reservoir through said bypass circuit in its activated condition;
• and
• second switchover valve means disposed within said braking circuit between said master cylinder and said first switchover valve means and being connected to said power pressure source through said bypass circuit to permit the flow of fluid under pressure supplied to said wheel brake cylinder through said braking circuit while said first switchover valve means is maintained in its deactivated condition and to interrupt said braking circuit and permit the flow of fluid under pressure supplied to said wheel brake cylinder from said power pressure source in response to a decrease of the pressure in said wheel brake cylinder caused by activation of said first switchover valve means;
• wherein said second switchover valve means includes a pair of valve seats opposed to each other to form a fluid chamber in open communication with said braking circuit and said bypass circuit, a valve element disposed within said fluid chamber to be selectively engaged with said valve seats, a control piston arranged coaxially with said valve seats and having one end to be applied with the braking pressure and another end to be applied with the wheel brake cylinder pressure, the one end of said control piston being provided with a rod portion in engagement with said valve element, and resilient means for biasing said control piston toward said valve element.