When that happened to me it was the master cylinder.
Still it is better to find out if it is the RABS or the master cylinder. Otherwise you start throwing parts at it like some mechanics. The last time I had someone do my rear brakes they wound up replacing the MC, Calipers, rear shoes, rear drums, rabs valve and rear wheel cylinders. This was over the course of two full weekends from open to close with me waiting. They also wound up replacing two wheel studs because the so called mechanic put the rear wheels on wrong on one side and when I pointed it out to him he proceeded to take the wheels off the one side and broke a wheel stud removing the nut.
After all was said and done I wound up getting every component replaced except the hoses and lines at no charge.
The last brake job I did myself and noticed that the self adjuster was installed backwards on the drivers side from when the mechanic did the brakes.