Ordinary Function
In ordinary brake operations, cut-off out valve in modulator is open to transmit hydraulic pressure from master cylinder to brake calipers via chamber A and chamber B. Chamber C is connected to reservoir through outlet valve which is normally open. It is also connected to hydraulic pressure source (pump, accumulator, pressure switch, etc.) via inlet valve which is normally closed. Chamber D serves as an air chamber. Under these conditions, pressures of chambers C and D are maintained at about atmospheric pressure, permitting regular braking operations. See Fig 1 .
If brake inputs (force exerted on brake pedal) are excessively large and a possibility of wheel locking occurs, control unit operates solenoid valve, closing outlet valve and opening inlet valve. As a result, high pressure is directed into chamber C, piston is pushed upward, causing slide piston to move upward and cut-off valve to close. As cut-off valve closes, flow from master cylinder to caliper is interrupted, volume of chamber B, which is connected to caliper, increases, and fluid pressure in caliper declines. When both of two valves, inlet and outlet, are closed (when only outlet valve is activated) pressure in caliper is maintained constant. When possibility of wheel locking ceases, it is necessary to restore pressure in caliper. Solenoid valve is therefore turned off (outlet valve, open; inlet valve, closed).