I recently went through the process of diagnosing and fixing my vacuum flush "head." It's not exactly the funnest of tasks, but, then again, neither is cruising without a functioning head. My top tip is to treat yourself to a vacuum gauge, and don't put it off like I did.
Full write-up and photos:
blog.arribasail.com/2015/04/tech-tale-of-two-heads-or-fixing-a-marine-toilet.html
Hey Scruzin, thanks for sharing the experience. I have a similar brand toilet. No problems yet but just wondering if your vacuum tank holds the goods and then you pump it to the holding tank at some time in the future. My vacuum tank is my holding tank and it has sensors for empty, 3/4 full and full. When Full, everything is disabled until the tank is emptied. I was hoping this feature may result in the vacuum pump never getting full of, um, um, $hit. Does yours have this cut out feature or will I also be going down the same path as you regards to the build up in the pump?
Hi Crusoe,
I have a holding tank gauge too, but it is just a sensor, and does not force the system to cut out.
I think you'll find only a small section, namely the pipe between the toilet and the vacuum pump, plus vacuum chamber, is kept under vacuum. The holding tank itself, which is downstream of the pump, is not under vacuum. That way pumping out to sea does not require the use of the vacuum pump, providing the outlet is below the holding tank. There should still be a macerator pump between the holding tank and the outlet though.
Thanks for the extra info on your system scruzin. Mine's a bit different from yours. It's called a Sailvac and has a tank that is shaped it go behind the wall in the toilet (ok head) cubicle up against the hull. It's not a big capacity tank but has been sufficient so far. The vac pump is quiet small and fits into the shape of the top of the tank. I have a seperate discharge pump located at the back of the boat that draws straight from this tank and discharges above the waterline at the stern. When discharging from this tank, I need to open the valve at the bottom of the toilet bowl to allow air to make its way back into the tank.
My toilet looks more like a ceramic one you find in a house and the hoses exit through the bottom. It has a small slide switch behind the toilet lid to manually overide the valve at the bottom. Stupid place to put it. In a rough sea, I have trouble getting my aim right so I relocated the switch to up on the wall with the rest of the controls.
The only problem I have had is people not making sure there is enough fluid in the mix when it evacuates the bowl. It has an add water button and I tell all concerned that it hates dry toilet paper. It's not a fun topic of discussion and new comers some times say they understand when they don't 
. Fortunately it's very easy to unblock 99% of the time. 
www.seabreeze-industries.com.au/products/SailVac-5200-Vacuum-Holding-Tank-554.htm
That looks like a nice system. It's great that it's all in one. The downside to my system is that you've long pipes and multiple connections between the head and the vacuum tank, all of which are susceptible to leaks, unless maintained.
