YES, your boats holding tank can be full.

I have had an absolute rash of customers with a similar Vacuflush problem. They come to me(Pflieger Marine Services 419-625-0605) and say " my vacuflush toilet won't make any vacuum but it does run" or "my Vacuflush toilet runs for a while and trips the breaker without making any vacuum".
I almost always tell them to check if there waste holding tank is full. The reply is inevitably, "it CAN"T be full, we just had it pumped out".

If your Vacuflush system is running without shutting off, and making NO VACUUM there is a good chance the tank is full or the vent line is plugged. When the Vacuflush pump comes on and pumps liquid to the waste holding tank, the air in that tank must be displaced out the vent line. If this does not happen the waste tank becomes pressurized. Once this happens the pump can not push against the back pressure thus no vacuum is made or(if the pump motor is weak) the breaker will trip. Worst case scenario is an exploded tank in the bilge.
If the vent line is plugged(from overfilling into it, corrosion or spider webs) then it can not be pumped out either. You may take the boat to the pump out and suck it out a little(usually just relieving the pressure on the tank) but with out positive airflow through the vent line the waste holding tank can not be pumped out. A trick is to hold down the foot pedal at the toilet thus giving a vent there.

The test is simple . Remove vent line from tank(carefully). Now flush toilet. If liquid pumps out of vent fitting, guess what? Now take boat to pump out (with this connection open) and pump out waste holding tank. After that check for a Sealand vent filter. If your boat is equipped with a Sealand vent filter, water in that filter will pack the activated charcoal thus stopping the venting action. Replace the Sealand vent filter. They may be purchased very inexpensively at my website www.marinediscounters.com. If no vent filter is present then check for water or T.P. in a low spot in the hose. Also look at the vent discharge fitting at the side of the boat. If it is a chrome fitting as is used on Vikings, egg harbor and ocean yacht the screen inside will corrode or be packed with spider webs and stop the venting action. As is always our policy, please feel free to call for free technical assistance at 419-625-0605 and we appreciate your business at www.marinediscounters.com.