Letting the black tank fill up sufficiently between dumps helps to get some head pressure for stuff to flow. When running the flush system it really helps to keep the gate valve closed to also get everything possible to float as well as build up a little head pressure. The way the black tanks on my Miramar 34.1 are designed they are long with a gradual incline from the passengers side of the coach that flows into a deeper sump on the drivers side where the dump valves and plumbing are.
The layout of the black tanks works better for the toilet on the drivers side of the coach because it is directly above the deeper sump portion of the tank. That puts the heavier waste where it needs to be in order to be dumped out.
However, The toilet on the passengers side of the coach is positioned over the shallowest portion of the black tank furthest away from the dumping valves and plumbing. Since that’s the case the heavier waste is potentially entering on the dry end of the tank and that will likely keep it from flowing down to the sump until there is sufficient waste built up in the tank. That’s why it helps to let the level build up and do some good tank flushing.
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