Could be air in the lines, or bad master cylinder.
To change fluid, buy FORD HEAVY DUTY ONLY !!! (was that clear?)
Then locate the bleeder screw on the caliper. Crack it open one revolution. When you pump the brakes, it will dribble out. Then you add new fluid to the master cylinder to top it off. Thats _basically_ it.
Get a Chilton's repair book for 10 pages of thorough explanation (versus my 2 liner).