Remove the entire chamber.
The
four screws on the main knob have been removed .You can now remove the
knob and unscrew the outer portion of the adjustment assembly from the
chamber.
Photo shows knob removed.
The outer portion which contained a piston has been
removed and now the chamber is open. You should be able to see a small amount
of fluid if you look down into the chamber. I am not sure what the fluid
is, so be careful, avoid eyes, do not breath etc. Wash hands. 
I
spread the spring slightly on my piston assembly to give a better feel.
The round item is the chamber cover.
The
open chamber is to the left. Use your thumb as a piston and try to force
a small amount of fluid out of the orifice that was exposed when you removed
the small cover with the rubber plug. You may run out of fluid which I did,
I replaced the fluid with alcohol. Fill the chamber about one forth full.
Alcohol is probably the wrong stuff but it worked. I do not know what the
original fluid was that was in the chamber, I tried light oil etc. but it
did not work. BE CAREFUL with the original fluid treat it with Caution.
Watch for the bubble to decrease. You do not have to put the chamber back on the sextant.
Once you get a bubble then put it all back together.