Conbraco 4050502 Vacuum breaker, repair help, Houston, TX

Hello, After the last round of freezes the vacuum breaker for my lawn sprinkler system needs replacing (extreme water leak under housing). It is a Conbraco Industries Model #4050502, SN CX886. I have taken the parts and this information to two trusted local hardware stores and they do not carry Conbraco, and the irrigation supply companies have also been a dead end for this brand. Trying to contact Conbraco through the web leads to an Apollo website that does not help. Before I give up and purchase a completely new unit from Febco, I thought I would try here. Does anybody know a company or website that cannot assist with Conbraco vacuum breaker parts? Thank you, Carl
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Carlshead wrote:

Sorry, can't help.
I've seen these things on lawn irrigation systems - can anyone explain what the heck they do?
http://www.sprinklerwarehouse.com/v/vspfiles/images/landing_pages/conbraco_backflow/conbraco_cdc4v_507_02_05.jpg
How is a "vacuum breaker" = "backflow preventer" ?
How exactly does the standing water in a sprinkler system overcome the high pressure of your average municipal water system such that you need a backflow preventer? These things are usually mounted up against a wall where your supply line runs out and down to your valve manifold. How the heck does the standing water that's under zero pressure flow up-hill and back into your fully pressurized water supply lines?
And what are the small ports on these things for (the things with the small red caps) ?
Going off on a slight tangent - are there any of these gadgets designed to draw in fertilizer or pesticide / herbicide from a jug (or sealed tank) and mix it with the water going to the sprinklers?
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Home Guy wrote:

http://www.sprinklerwarehouse.com/v/vspfiles/images/landing_pages/conbraco_backflow/conbraco_cdc4v_507_02_05.jpg
I haven't looked real close, but I think this kind of vacuum breaker is used (here) on commercial buildings and cheap plastic ones are used on residential.

If you get a negative pressure (siphon) on the water pipe the vacuum breaker opens to air, so water is not siphoned.

The same way water can be sucked out of your toilet tank. Probably very common to require toilet 'ballcock' valves to be antisiphon. Also required clearances between faucets and sink overflow levels to prevent siphon. And hose connections may require vacuum breakers. It is one of the basic principles for plumbing systems.
One way a siphon could happen is if there is a fire, and pumping city water creates a negative pressure (and you flush the toilet). Another could be if the city water is turned off (as for repair) and water drains back downhill or into a hole creating a siphon.

I believe they are for testing.

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