The ABS Control Module, or ECU, sends commands in the form of electrical signals to the hydraulic control unit. This unit executes the commands, using three solenoid valves connected in series with the master cylinder and the brake circuits - one valve for each front wheel hydraulic circuit, and one for both of the rear wheels.
In normal, non-ABS braking, brake pedal force is transmitted to the master cylinder, then through the solenoid valve to the brake unit at the wheel. When the signals from the wheel speed sensor show no tendency for the wheel to lock up, the ECU does not send any control current to the solenoid coil. The solenoid valve is not energized, and the hydraulic pressure from the master cylinder is supplied to the brake unit at the wheel.
When the control unit detects any lock-up tendency, perhaps from too-rapid wheel deceleration, it sends a command current to the solenoid coil. This causes the armature and valve to move upward, and isolate the brake circuit from the master cylinder. That keeps the pressure between the solenoid and the brake circuit constant - whether or not the master cylinder hydraulic pressure rises.
If the sensors signal continuing excessive wheel deceleration, the Control Module sends a larger current to the solenoid valve. This lowers the braking pressure by moving the armature up further, opening a passage from the brake circuit to an accumulator - a temporary reservoir for any brake fluid that flows out of the wheel brake cylinders because of the fall in pressure. A return pump sends this brake fluid back to the master cylinder.
If the sensors then signal that the lower pressure has allowed the wheel to speed up, the ECU stops all command current, which de-energizes the solenoid valve. The pressure rises, and the wheel is again slowed down.
Whatever the phase of operation, pressure in the circuit can never rise above master cylinder pressure.