One thing that’s happened to me a couple of times on older cars is that the float sensor in the master cylinder, which measures brake fluid level, gets stuck. The brake fluid drops when you step on the brake, but then when it comes back up, the sensor sticks in the lower position and fools the system into thinking the fluid is low.
The fix is to clean the rod that the float sensor rides on - it’s full of gunk.
Also, when’s the last time you had the brake fluid exchanged?
IMO the ABS warrants immediate attention if for no other reason than if you happen to get into a wreck and someone finds out that you knew the ABS wasn’t working, that could expose you to legal liability.
For the other thing, check that float sensor first, and change the fluid if it hasn’t been done in the last 3 years. Then see if it keeps happening.