self adjusting No1 MkIII rear sight

[infideleggwelder]

on my No1, the rear sight resets itself as far back as it can go with the first shot fired. didn`t notice this until a week or so ago, and was starting to doubt my shooting. set sights for 200, fire one shot, sights are back to "0" as it were. reset to 200, fire, sights back to "0". it has been suggested that the spring inside the rear sight may be old and tired.
i reconfirmed my shooting with a 6 inch horizontal group at 200 with my No5 with iron sight.
any thoughts on the No1?

 

[Blastattack]

Something is broken. The slider is spring loaded and a threaded wheel/adjuster should lock into the threads impressed into the side of the sight.

 

[John Sukey]

Possibly a good dose of break free would help remove the dried grunge in the slider.

 

[infideleggwelder]

everything appears to be in order, everything moves, locks, slides,etc. just dosen`t stay put when fired. i`ll see if cleaning it will help. can`t make it move unless i fire it.