You need to suck out all the old fluid from the master cylinder,using a turkey baster. Then refill the resivoir with clean fluid and rebleed your brakes in the propeer order,rt rear,left rear,rt frt,lft frt. Bleed each caliper until clean fluid flows out ,make sure you keep the master full during this procedure. The clean fluid should free up valves if they aren't to badly clogged, if not you will prob have to have your ABS serviced by a shop.
Brake fluid should be changed about every two years to keep your system clean,because it absorbs moisture, which forms in the calipers from the heat of braking.
I bleed my brakes once a year with clean fluid and anytime I do a brake job I replace all the old with new fluid and bleed the brakes. Good luck