^ +1 for you to check all of what Osei posted.
In addition to that, you should flush your brake fluid if it hasn't been done in the past year or two.
To check if it's the ABS, you should pull the ABS fuse and see if it still acts up. Pulling the ABS fuse will disable the component until you put the fuses back in.
-- Thread Here
'13 Prius Five