Avid Juicy 3 brake piston problems
You absolutely should not have let fluid out of the system. If the brake was working before there was not too much fluid in the system. You will need to bleed the brake to remove air bubbles and restore the fluid level. (If the back brake has not been bled in eight years you should do that one too.) Dirty pistons and seals are usually to blame for non-retracting pistons. In extreme cases the whole caliper has to be disassembled and cleaned.

You can try to find the manual for Avid brakes by searching on the SRAM website. There is a 2006 manual which may apply. 

Check out this Park Tool video for how to clean hydraulic brake calipers. It also explains how piston retraction and advancement works.