After years of doing the two person bleeder routine I broke down and spent $35 on a vacuum bleeder at Advance Auto. Once I learned to use it I kick myself in the butt for not doing this years ago.
Basically I flush brake fluid every two years as a preventive maintenance whenever I do a brake inspection. Its more time consuming but in the process I know my calipers are function properly and all the gunky stuff and moisture is out of the piston chambers. Assuming you are going in with new pads when you change calipers, and everything glides on the frame, and you know the calipers are moving....if after all that you still have little brake grip I'd be looking at vacuum leaks as @montanna tom has suggested.
Jeff