5 second vacuflush kerplunk

[oceanpearl]

My guest head cycles for about 5 seconds intermittently like every 2 hours.
However it cycles for about 20 seconds upon turning on the breaker after a couple of absentee days. The bowl stays full of water so the leak is not caused by the seal.
I know there is a leak somewhere and hating hit and miss mechanics, can anyone suggest a possible cause.

 

[Boater420]

I'd start with the duck bills if you don't know when they were last changed. If they're relatively new, you might try flushing a few bowls of water through the system first or try plunging lightly to remove any TP that might be stuck in the duckbill.

You can also go down into the bilge and listen for vacuum leaks right after the pump cycles off.

 

[oceanpearl]

Thanks Kevin.
I'll give your suggestions a try. Much appreciated.

 

[dtfeld]

I just went through this, it was the shaft o-rings on the ball valve shaft. I put my ear next to the toilet and you could hear the vacuum leaking. Water remained in the toilet but only indicates if the ball valve seal is sealing. The shaft o-rings are underneath and can leak without the water leaking down.

Dometic get $$$ for the assembly ball assembly. 2 o-rings would be $0.50 if you can measure them. I ended up replacing the duck bills, the ball seal and the ball valve/shaft assembly in that order. I can be gone 2 weeks and the pump will not cycle, so I think money well spent.

 

[Rssll45]

There are many threads posted on this subject. Before you go pulling the vacuum generator, replacing duck bills, etc. locate the leak by sealing off the pump in different places. One of the first places to look is the elbow coming out of the tank-a simple fix, but its easy to seal off the pump at both ends and will save a lot of unnecessary work.