This old thread helped me recently when I lost the lever to my pre-1999 3021, so I'm giving some back with an alternative fix.
Unfortunately, even after sourcing and receiving a new lever (from manfrottospares.com), the center column could not get as tight it should. When I opened up the top cover of my tripod, I discovered that my "lock pin" which is the crescent shaped plastic that gets pushed into the center column was broken.
I couldn't simply replace it as the old lever mechanism has a single spiral that mated with the back of the "lock pin". The new lock pins available now doesn't have anything on the back to mate to the single spiral of the locking bolt mechanism so turning the lever does nothing to push the lock pin against the center column.
I sourced the following parts: 1/4" Draw bolt (used to join countertops, I'm using only the threaded rectangular piece); a 1/4" ratchet lever bolt (used for woodworking/drill press clamps); a piece of high density polypropylene I hand shaped (with a 1-1/4" hole saw and a molding cutter/fixture), and misc parts that include a washer & spring.
Check out the attached pictures. Hope this helps someone in a pinch.
