A pressure toilet has a trapway providing “as needed” (bulk dependent) siphon assist. During normal liquid waste or low bulk flushing, no siphon is formed in the trapway, and the water and light waste in the bowl are evacuated solely under the force of the pressurized jet of water. A large volume near or just downstream from the dam is provided to accommodate the blow out from the water jet. Only upon reaching a threshold concentration of bulk waste material in the down leg does the trapway draw a siphon. A horizontal baffle at the lower part of the down leg assists in the accumulation of bulk material of sufficient concentration to establish a siphon in the trapway.