My compass developed an air bubble (which seems to only be there on colder days, I guess the oil inside expands/contracts with temperature).
Is this something I could theoretically repair myself? The compass is as old as the boat and certainly not worth repairing commercially (I'd rather buy another one if that's the case)
This is something you can do. Over the Winter, I refurbished my 5" Danforth Constellation. It was doing the same thing. Many of these old compasses are fine machines and are well worth the trouble. This particular design has been manufactured by one company or another since the late '40s. I spent several hundred on parts and oil, but a new compass of similar quality would be at least 4 times that.
I'm a computer engineer and work for a defense contractor, so it doesn't matter to me who you use. However, I will pass on that I had a good experience buying parts and receiving good advice from Viking Instruments
. Howard there is quite knowledgeable.
From personal experience I can tell you that if you have a leak, eventually the diaphragm collapses as much as it can and then the compass inhales air as it cools. Finding the leak is hard. The oil is volatile and evaporates as fast as a slow leak leaks, so you won't be able to locate it by looking for a wet spot.
I will pass on a few things from Howard ... buy compass oil, not mineral spirits, unless you are adding just a little bit to top up. If you disassemble the compass and replace all the liquid, he said that mineral spirits will work but will eventually darken as it leaches color from rubber parts. I'm not sure that's true, but I pass it on FWIW. In addition to Viking, West Marine and many others carry compass oil. Second, if you replace parts, getting the compass reassembled so it is tight can be tricky, depending on the design. Don't be afraid to ask for advice. If you have to replace parts, then once you have the compass together, filled, and you think it is tight, put it in the freezer to see if it inhales air.
If your leak is slow enough, you want to take the easy route and simply put more oil in rather than finding and fixing it. Remove your compass from its mounting and look for a screw with a rubber washer under it. Orient the compass so that the screw is on top then remove the screw to add the oil. I found a nasal aspirator for babies works pretty well for the task. Before you put the screw back in, jiggle and roll the compass around a little bit to get as much of the bubble out through the screw hole as you can. It's also a good idea to make sure the diaphragm has a little slack in it so that it doesn't rupture from expansion on hot days. Poke a finger in the diaphragm as you tighten the screw and push a little oil out as you close it. Needless to say, this operation can be messy. I did mine in a dish pan.