Black tank flush vacuum breaker leak in 388m, but only with certain sequence

[Cc3]

We recently purchased a Momentum 388m (with rear half bath in garage) and we are having an odd problem with the black tank flush for the main front bathroom. It is leaking at the vacuum breaker when in use. But, the strange thing is that the leak only occurs when we use the rear black tank flush first. We have replicated it twice,but it only happens when the rear flush is used first...what could be causing this?

Here is the exact sequence:

Incident 1:
1. Dump and use blank tank flush for rear half bath. No leaking.
2. Dump and use black tank flush in main bathroom, leaking at vacuum breaker within 5min.

incident 2 (1 week later):
1. Dump and use blank tank flush for main bathroom. No leaking.
2. Dump and use black tank flush for rear half bath. No leaking.
3. Go back to front tank (sensors still reading full and thought since it wasn’t leaking, could try more time), use black tank flush, leaking at vacuum breaker within 5min.

Looking at maintenance records, it might have been happened a couple times since new, but dealer could never replicate it and I’m guessing since it only happened under certain conditions, previous owners didn’t encounter it too frequently.

We could always flush tanks in order that seems ok (front first), but I would like to fix whatever is causing this if possible. Or, is it a known thing that the front flush needs to be used first?

[howson]

We recently purchased a Momentum 388m (with rear half bath in garage) and we are having an odd problem with the black tank flush for the main front bathroom. It is leaking at the vacuum breaker when in use. But, the strange thing is that the leak only occurs when we use the rear black tank flush first. We have replicated it twice,but it only happens when the rear flush is used first...what could be causing this?

Here is the exact sequence:

Incident 1:
1. Dump and use blank tank flush for rear half bath. No leaking.
2. Dump and use black tank flush in main bathroom, leaking at vacuum breaker within 5min.

incident 2 (1 week later):
1. Dump and use blank tank flush for main bathroom. No leaking.
2. Dump and use black tank flush for rear half bath. No leaking.
3. Go back to front tank (sensors still reading full and thought since it wasn’t leaking, could try more time), use black tank flush, leaking at vacuum breaker within 5min.

Looking at maintenance records, it might have been happened a couple times since new, but dealer could never replicate it and I’m guessing since it only happened under certain conditions, previous owners didn’t encounter it too frequently.

We could always flush tanks in order that seems ok (front first), but I would like to fix whatever is causing this if possible. Or, is it a known thing that the front flush needs to be used first?

I can't even guess how the two systems would be connected. I seriously doubt they are connected and the leaking based on what-is-flushed-when is coincidental.

Below is an example of a typical black flush setup:



The moderators do not have any diagrams for the 388M, but even the diagrams we have with similar floorplans (like the 381M) do not show this black flush system.

So help me understand the setup. Are there two separate black flush connections on the exterior of the coach (where you hook up the water hose) for the two black tanks? Pictures would be *great* if you can figure out how to upload a few.

Howard

P.S. Regardless of how and why the valve is leaking, re-crimping the water lines to the valve or replacing the valve should be a relatively easy task even if you're a basic DIY'er.

[JBill9694]

Curious as to how Cc3 is using the "flush". Is the flush done with the dump valve open, or is the valve closed and the water metered or timed to partially fill the tank before opening the valve? If the latter, might want to reduce the volume or time.

Also, curious about the water pressure used for the flush. Although a PRV is not typically used on the flush system too high a pressure could result in the leaks at the connections to the vacuum breaker. (A good reason to follow up on howson 's advice.