Black tank doesn't fill

[MrBill]

First, my gauges are not reliable for indicating how full my tanks are and I thought my black tank was starting to "burp" before it should have. So I emptied the tank, put a water meter on and measured how many gallons of water it took before the commode started burping again. Seems my tank is getting only half full. I'm guessing there is a problem with my vent pipe but I don't know what or how to check it.

 

[Birchwood]

Our black water tank used to "burp" for some unknown reason and it didn't matter what the water level was in the tank.
Even after it was emptied and refilled a bit it was still doing the same.
I added more water and reflushed and it disappeared and has been ok since.Never really
determined what the cause was but has been ok for several weeks .

 

[irvin56]

My dealer mentioned he had seen this before. The piping going into tank was too long. once it was taken out and shortened it stopped the problem. I would look at that as maybe the reason. Just my .04(inflation)

 

[HappyKayakers]

You might also be building up an 'obstruction' directly below the pipe. I read on another forum that after dumping the black tank you should fill the bowl completely and flush a few times. Theoretically this should break up any budding pyramid in the tank.