My 2003 Adventure does the same thing if I let it sit for more than two or three weeks. As far as a permanent fix, I haven't found one. Air must be creeping into the caliper somehow. Two tricks that have helped me are:
1. First, I purge the caliper by carefully and firmly squeezing the piston side of the caliper towards the disc, thus creating a gap between the opposite "fixed" brake pad and the disc.
2. Next turn your handlebars to the left, raising your fluid reservoir to the highest point. Hold the upper curve of the brake line down to create a downwards run from the reservoir to the caliper, then begin squeezing the brake lever slowly, and firmly to bring the air bubbles up. You could even have the cap off of the reservoir at this point to check the air release, and monitor the fluid level.
A lot of times I will just do the second step, and it will make things rock solid again.
2003 KTM Adventure 640