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Old 10-30-2018, 07:16 PM   #1
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Weird plumbing under bathroom sink

2001 Monaco Diplomat

While changing out the faucet in the bathroom, I discovered the attached connection.

Anyone know what it's for?
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Old 10-30-2018, 07:22 PM   #2
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Looks like the anti-syphon valve for the black tank flush system....keeps bad water from drawing back up pipe to potentially mix with good water....
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Old 10-30-2018, 08:31 PM   #3
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Oldscout is correct. I changed one on my old travel trailer, $75
If the flush out is working and there is no oder under the sink, and no leaks, leave it alone.
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Old 10-30-2018, 08:35 PM   #4
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Some people call it a back flow preventer.
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Old 10-30-2018, 09:21 PM   #5
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...or a "vacuum breaker" value...purpose is the same--keeps bad water from mixing with good water.....
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Old 10-30-2018, 09:45 PM   #6
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So when it starts leaking, there are brass replacement ones thru HD....
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Old 10-30-2018, 10:40 PM   #7
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It is a combination vacuum break valve (with an internal poppet valve that allows atmospheric air into the line if it tries to siphon black tank water out to the fresh water supply) and a downstream backflow preventer valve (also prevents black tank water siphoning out to the fresh water supply and also prevents black tank fumes from coming into the vacuum break poppet valve and leaking into the coach).

Sooner or later, that cheap plastic valve will die and black tank sewer gas will come up under that sink. Proactively replace with new brass vacuum break valve and new brass back flow preventer to prevent that from happening.
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Old 10-30-2018, 10:55 PM   #8
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Quote:
Originally Posted by bob caldwell View Post
So when it starts leaking, there are brass replacement ones thru HD....

Assuming that you mean Home Depot, yes they are
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Old 10-30-2018, 10:59 PM   #9
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Anti-siphon vacuum breaker back flow preventer valve



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Old 10-31-2018, 10:06 AM   #10
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I replaced the cheap plastic factory combination valve with a new brass vacuum break and new brass backflow preventer valve.

In this picture of a standard cheap plastic factory combination valve, you can see the vacuum break valve in the main body and the backflow preventer in the output side. Click image for larger version

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Here is the replacement vacuum break valve and backflow preventer valve. These should last for the life of the coach.
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