did you use the right brake fluid? if you happened to mix dot5 with dot 3 or 4 brake fluid you will get this problem < dont ask me how I know >. If you have an old can of brake fluid lying around your garage it is possible that it absorbed water also.
also check to see if your rotor is bent and / or your caliper sticking. worn/rusted brake pins?
Did you happen to be jumping and possibly land with your foot on the brake pedal recently maybe the little rod that goes into the master cylinder is bent or adjusted too tight. Is the brake return spring on the pedal?
time too scrutinize each and every part individually and also how each part works in the assembly . If anything is questionable or you are unsure of it toss it and get new, brakes are definately one area you don't wanna play cheapskate with