Please look at the brake fluid in the master cylinder. It should be very clean and free of any particles. The presence of any dirt or black particles would be a sign of the master cylinder failing.

As noted at many places on the site the control modules are reported to fail by many people. If the control module fails you will have NO ABS function and will not be as safe, but the basic operation of the braking system will not be affected. People drive their cars without the control module while having it repaired by Rocha.

Since you have other symptoms, not just the ABS light on, it seems likely there is something else going on (maybe the ABS module is also bad). Did the car have other brake work where the system may have been opened up, not filled and bled well, or gotten some contamination in it?

I don't know what Volvo recommends on replacing brake fluid - I have developed the policy of replacing it every 3-4 years. Once contamination or dirt gets in it, for instance from a bad seal, it needs to be removed or it will spread around and affect the whole system (including the ABS hydraulic pump) and you may be in for repeated repairs.