Vacuum breakers like this one are what feeds the pools auto fill of the pool. It is a one way valve, that is designed to prevent water from being siphoned back into the public drinking water system. Without it, in the event the city's water pressure drops, the drinking water could become contaminated. The blue valve handle that controls water flow is open, allowing water to pass to the pool deck, when inline with the pipe. It cuts the water off to the pool deck when it's turned 90 degrees and not in line with the pipe. I usually see 3/4" PVC pipe coming off of a vacuum breaker, but in the image above, it is a copper pipe.  

The water from the vacuum breaker valve goes to the pool deck, to a float valve like in the picture above. As the water level in the pool drops, so does the valve. This opens the valve, allowing water to pass and thats what fills the pool. This is necessary to stave off evaporation.

Inside the vacuum breaker, there is moving parts. There's a spring, and the other parts are made of plastic and rubber. When the water in the valve body freezes, it damages those parts and the valve will leak. Rebuild kits are available at sprinkler supply stores, but often, the plastic cap cracks, and you will have to buy the whole valve complete to get the cap. In that case you use the guts of the new valve, to rebuild your existing valve. 

Covering your vacuum breaker with towels can insulate it and help it from freezing. Rebuilding a valve can save you about $50-$70 for new parts, possibly as much as $140 to $160 if you pay someone to fix it for you. Its worth it if you know there's going to be freezing temperatures. 

Update 07.07.2017

The best way to prepare the vacuum break for a hard freeze, or if you live in a state that has a hard winter, is to cover all of the exposed pipe with Polyethylene pipe insulation. Shut off the water to the vacuum breaker. Next, loosen the screws on both test ports on the side of the breaker. Allow the water in the vacuum breaker to drain out. This will keep it from freezing, expanding and ruining the internal parts. 

As good as these cleaners are, parts do wear out. Cleaners run under pressure, 6-10 hours a day, each and every day. They are work horses and parts will wear out over time. After 1 year on the older Poolvergnuegen cleaners, and two years on the new generation cleaners, expect to start needing to replace some of the parts. Parts are available if needed separately, however, Hayward now offers a lower propulsion kit in both the 2 wheel and 4 wheel versions for a fraction of the price of a new cleaner, or replacing the parts purchased separately. The lower propulsion kits come complete with everything except the skirts, tires, turbine cover and top body cover. In the picture above, you see a lower propulsion kit for a 4 wheel cleaner. I like these cleaners. 

Tires are the most common part to be replaced on these cleaners with in the first two years. Tires must be replaced once they  reach the wear markers. The wear markers are raised bumps as shown above in the screen shot from the owners manual. The tires can wear faster in pools with a rougher surface like 3M Quartz and Pebble Tech, but will wear down to the markers with-in a year. Allowing the tires to wear down past the markers too far, will put extra stress on the internal parts and causes bearings, drive shafts and gears/teeth to wear out prematurely. I check the tires every 6 months. Excessive wear of internal parts with in the warranty period as a result of negligence [not replacing the tires] will void the warranty.
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