If it's a Ritchie compass, parts are available. L Kieth is right. You have a leak. Ritchie sells a new diaphragm for about $11. Older ones use oderless mineral spirits. Newer ones use alcohol, I think. Put a new diaphragm in, fill 3/4 or so with mineral spirits, put compass and extra mineral spirits in the freezer overnight then top off the next morning. I used that technique 3 years ago and the bubble has not reappered.