If any of the solder joints fail or the capillary tube fatigues and develops a crack the ether trapped in the expansion bulb and tube will leak out and the gauge stops working.
They are repairable either professionally or as a DIY project. The only caveat against a DIY repair that I will make is as follows. IF the gauge dies where the needle is not resting on the lower stop peg, then more is required than basic repair. Such gauges should be left to the pros.
If on the other hand the needle is resting on the lower stop peg and the gauge just stops working... use the DIY repair method discussed in the link below. HOWEVER, I suggest you use dry ice for the repair instead of the salt/ice cold bath. Dry ice cooling is much colder, cleaner, and does not result in any possible corrosion of the capillary tube's protective spring.
DIY Gauge Repair Link