Field strip the slide (remove barrel).
Pour enough lacquer thinner in a cup or jar so as to just cover the sight when the slide is stood in it.
(Best do this OUTSIDE to prevent fire danger).
As it soaks, remove and move the sight around then soak again.
After soaking allow to air dry until most of the thinner has evaporated, then warm the metal with a hair dryer to finish drying.
WARM not HOT.
After the slide is totally dry, inject some Loctite Red (permanent) into the sight dovetail.
Center the sight on the slide. Wipe off the excess,and allow 24 hours to cure.
Apply a lube to prevent rust.