Keep It Cool -- Gas Refrigerator Basics 

Gas refrigerators are entirely different from their electric cousins. Where electric refrigerators use safe, non-flammable freon for the refrigerant, gas refrigerators use compressed hydrogen gas and an ammonia-water solution. Electric refrigerators use compressors, electric motors, and cooling fans; gas refrigerators have no moving parts.   Finally, where electric refrigerators use copper tubing, gas refrigerator cooling units are made entirely of steel because of the ammonia. Ammonia attacks copper, brass, and bronze in the presence of water. You can see that the repair of gas refrigerators should never be attempted without extensive training in safety procedures.

How They Fail
The single biggest killer of gas refrigerator cooling units is corrosion--plain old rust. The common misconception is that it's the rust on the exposed pipe at the bottom of the cooling unit (down by the burner) that causes the leaks. In fact, this is rarely the case. The rust often seen on the exposed pipes is largely superficial even though it may look terrible. In gas refrigerator cooling units, the worst corrosion always occurs on the evaporator piping (the pipes that get cold) because they are physically located inside the refrigerator cabinet (right behind the back wall of the interior) and are covered by insulation. While the refrigerator is running, moist air finds its way to the cold evaporator pipes and condenses on the surface. Since the evaporator pipes are encased, the condensed water cannot readily drain away and therefore is held next to the steel pipe. After several years, rust develops.

The rusting is accelerated when the refrigerator is turned off for the season, allowing the evaporator to warm up. The rusting eventually progresses to a point where severe pitting occurs in the surface of the steel. These pits eventually extend all the way through the pipe wall into the inner space of the pipes. The result is a leak.  You will usually, but not always, smell the leak when it occurs. If the leak does occur on the outside piping, you probably won't smell it. Sometimes, even if the leak is on the inside, you may not smell it if mostly hydrogen is leaking out. Most of the time, however, you will know that your refrigerator is leaking because of the pungent ammonia smell coming from inside your cabinet. 

After your refrigerator starts leaking, it will eventually (usually very soon) stop cooling. The worst thing you could do at this point is to delay in getting your refrigerator repaired because corrosive ammonia is eating away at the cooling unit steel (ammonia doesn't corrode the steel while it's inside the cooling unit because of the rust inhibitor in there). The longer you delay getting your refrigerator's cooling unit rebuilt, the lower the odds are of a successful and reliable repair. You can see why removing all rust from the cooling unit by sand blasting and rust-proofing the cooling unit are essential steps in a proper cooling unit repair. Just as with your car, if you don't remove the rust, it'll just come back and, after another season or two of use, the cooling unit will fail again.