Since I had no instruction sheet, breaking them down was a matter of trial and error (as usual). First thing is to locate a small and hidden screw artfully recessed that prevents the lever's pin from sliding out through use (a smart and thoughtful design). Once located in the inside of the lever's body you need a 2mm Allen to unscrew it off.
Now it's time to clean, add some lube on the moving parts and reassemble it the reverse way.
Just a minor drawback: there's no build-in quick release (though none contemporary models had them, as far as I know...)