If it's been at least two years since the duckbills in the vacuum were replaced, it's a good bet that's where the air leak is...replacing 'em will solve the problem.
But first, I'd try flushing
at least two FULL bowls of water through it. I may be that there's just some waste or TP stuck in one of the duckbills that's keeping it open. That happens a lot to owners who believe that V/Flush doesn't need more than a pint of water/flush.
If the bowl holds water, it can't be the bowl seal or the dome in to the bottom of the bowl.
And while a vacuum tester gauge is dandy tool, it's a PITA to use to find an air leak in a V/Flush system. There's a MUCH easier way to hunt down an air leak: Squirt a LITTLE shaving cream onto each hose connection...flush the toilet. The suction will pull the shaving cream into any connection that's leaking...it'll just sit there on all the ones that aren't.
From your description of the vacuum tank as "big cylinder thing," I suspect you don't have an owners manual for it. You need one...'cuz it includes a trouble shooting guide that covers just about every possible symptom that a V/Flush toilet can develop, the possible causes and cures. You can download and print one from the Dometic SeaLand site here: VacFlush Owners Manual
And it doesn't matter how old yours is, 'cuz the VacuFlush hasn't changed enough to notice, except cosmetically, since it was first introduced by Mansfield in the 1970s...so the current
manual's trouble shooting guide is just as good for a 20 year old V/Flush as it is for a brand new one.
Or...I see you're in Annapolis
...the sailboat show starts on Thursday. Invest in a ticket and spend some time in the SeaLand Dometic booth learning
how a VacuFlush works (there is no macerator in it) You can pick up an owners manual there too.
Meanwhile, this may help:
VacuFlush 101, Part 1--How it Works:
It's a 3 component system--the bowl assembly, the vacuum accumulator tank (not to be confused with a holding tank) and a vacuum pump. The vacuum tank and vacuum pump may be a combined gizmo called the "vacuum generator
," but it includes both and works the same way as separate vacuum tank and pump. The holding tank
is NOT part of the VacuFlush toilet system, and neither is the tank overboard
discharge pump...SeaLand just bundles holding tanks
and discharge pumps with the toilet system.
The vacuum pump has TWO functions: it suctions the air out of the plumbing
between the toilet bowl and itself while simultaneously pushing the flush the rest of the way to the tank, treatment device or thru-hull. The vacuum tank has a switch and a sensor on it that starts the vacuum pump when there's a loss of "vacuum" (toilet is flushed or an air leak in the system) and turns it off when the correct amount of negative pressure has been reached. How long it runs depends on the distance from the pump to the bowl...the shorter the distance, the shorter pump run time. The accumulated "vacuum" only pulls the bowl contents TO the pump..the pump has to push it the rest of the way.
are the only thing that will cause the vacuum pump to cycle between flushes for no reason. The most common source of air leaks is worn out duckbill valves or something (waste or toilet paper) caught em, in the vacuum pump 4 of 'em--2 in, 2 out...so if it does start cycling for no reason, first thing to do is flush a full bowl of water, even two bowls full. If that doesn’t cure the problem, replace 'em. They should be replaced about every two years as preventive maintenance
anyway...even annually if you live aboard.
As duckbills become more worn, the pump will cycle more often and will run longer and longer each time. A failing vacuum tank switch can also cause the pump to run longer--till eventually it won't turn off at all unless you turn it off at the breaker--but that will NOT cause the pump to cycle for no reason.
VacuFlush 101, Part 2 Actual Flush Water Requirement
The two most common mistakes
that V/Flush owners make are:
1) Easing the pedal back up instead of just letting it go.
It's spring loaded for a reason--to snap the dome back into place it with enough force to seat it and seal it. So just let it go! If you don’t, over time you’ll develop an air leak in the bowl.
2. Using too little flush water.
SeaLand's claim that the V/Flush can use "as little as" 1 pint of flush water is VERY carefully worded. If only urine is flushed...no water added to the bowl first, no TP either...then yes, you CAN get away with that little, at least for a few flushes. But if you do much of that, unless you want odor
, at least once a day you need to run at least half a bowl of clean water through it to rinse out the system. It's also advisable to add at least half a bowl of water ahead of solids or any TP (iow, every time a female uses the toilet)...that's a quart or more. It's also essential to leave the pedal down for at least 7-10 seconds after the bowl is empty to rinse out the pump and duckbills--to prevent a buildup in the pump or bits of waste or TP from becoming stuck in a duckbill, creating one of those pesky air leaks that causes the pump to cycle for no reason...at least another quart. And if you don't want permeated hoses, it's a very good idea--last thing before the boat will sit (or at least once a week if you're living aboard)--to fill the bowl to the rim with clean water and flush it through to thoroughly rinse out the vacuum tank, hoses and pump...'cuz suction splatters waste all over 'em and the flush water flow isn't sufficient to completely fill the hoses. So, averaged out over a week, the VacuFlush actually NEEDS about the same amount of flush water as most other toilets that use pressurized flush water: about .5 gal/flush. If you're using much less than that, you're asking for problems.
Btw...I had V/Flush toilets on my last two boats and was also a dealer for nearly 10 years...so I'm INTIMATELY acquainted with 'em...what keeps 'em working trouble-free and what doesn't.