Model number for a VacuFlush only refers to the bowl size/height/discharge fitting location...the rest of the system is the same for all models.
There are several possible reason why the vacuum pump would be sluggish or trip the breaker:
1. Low voltage
2. Improper wire size
3. Discharge line blocked, causing back pressure
4. Plugged or defective duckbill valve(s)
5. Blocked holding tank vent
6. Worn or defective gear motor
7. Clogged bellows in the vacuum pump.
1 and 2 are highly unlikely in a system that isn't brand new and incorrectly installed. #6 is unlikely unless it's at least 10 years old.
That leaves a blocked tank vent or a clogged vacuum pump--either the bellows or the duckbills or both. You can learn all about holding tank vents, why vent line filters are a VERY bad idea and how to clear blockages and prevent 'em in the "holding tank vent" thread this very week.
If it's not the vent, it has to be the vacuum pump...and you're gonna have to take it apart to clean it out. If someone hasn't flushed something they shouldn't have, it's a certainty that you aren't putting enough water through it. So here's the short course in TROUBLE-FREE VacuFlush operatation and maintenance
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!
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. 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.
If the PO wasn't smart enough to keep the owners manual, call SeaLand and get one 800-321-9886. It includes just about every possible symptom, possible causes and cures that a V/Flush can develop. Doesn't matter how old yours is, the VacuFlush hasn't changed enough to notice since it was first introduced in the late 1970s...so a new manual will work just as well for trouble-shooting as the one that came with yours.