Back in March was it? we retrofitted a composting toilet setup out here, by installing a huge Phoenix compost bin in the greenhouse and a small Sealand marine macerating toilet into the tiny bathroom, which pumps up and over to the compost bin. This toilet has a sort of ball valve in the bottom of the bowl. The toilet is no longer holding water, the valve hasn't been sealing. The thing came with a little seal-cleaning brush - I tried this according to the instructions and it didn't work.
Gloving up, I determined the problem. The valve was not closing all the way. It needs to tuck under the seal but was stopping at the edge of it. This allowed water to leak down and gas to leak up. Nice.
As a temporary workaround I found that spraying the valve with Pam allows it to close properly, at least for a while. There may be a looseness problem between the valve and the pedal that actuates it. Maybe I can talk Pigpen into looking at it, I've had enough of the dratted thing for now.