ABS modulator solenoid valves open and close to control brake pressure. There are typically two valves per wheel: an inlet valve and an outlet valve. The inlet valve blocks fluid from the master cylinder, while the outlet valve releases pressure to the accumulator. The computer cycles these valves rapidly to keep the tire rolling.
Dirty brake fluid is the main enemy of these valves. Tiny particles can clog the valve seats, preventing them from sealing. This leads to dropping brake pressure or a dragging caliper.