If they are Maralon I wouldn't try much at all except the intense looking mentioned above
.... since they are very cold and brittle now.
If they are bronze... I tried when I had a similar problem ..... some WD40 which may (if the shaft is frozen) or may not do any good from the inside, a bit of heat from an electric paint stripper gun, and some light tapping. Even that only got me limited movement which I fixed at next haul by spraying WD40 up into the valve from outside.