It doesnt work that way
I had precisely this, both from the indaquate cistern to bowl and bowwl to soilpipe connectors.
Mine were neoprene, and I carefully lifted the outer section, drained and dried them, and injected clear silcone into th first 'chamber'.
Whether or not 'foul water' is trapped behind any other parts I neither know nor care. Its below water level on th soil pipe, and its clan water at tee bowl entry. So no smells.