The ABS modulator accumulator holds fluid when the system releases brake pressure. When the wheel speed sensor detects a slide, the modulator opens a valve. Fluid rushes into the accumulator to drop pressure at the caliper instantly. Then, the ABS pump moves this fluid back to the master cylinder reservoir.
Accumulators use a diaphragm and nitrogen gas chamber to cushion the fluid. If the diaphragm ruptures, the accumulator fills with fluid and stops working. This makes the ABS pedal pulse feel harsh or delayed.