Abs System
WARNING: This page is about a different car, the 1996 Volvo 850. However, it is still accessible from the selected car via links, so may be relevant.
If wheel lock-up is detected during braking, ABS control module closes inlet valve to brake circuit concerned, stopping supply of brake fluid from master cylinder. This prevents hydraulic pressure from increasing, regardless of pressure in master cylinder. Hydraulic pump starts up at same time.