Same thing happened to me. It just pushes in, sort of some sort of gasket. I applied some silicon grease to the pipe to assist the job. Been about five months now, all is well.
Slight correction I didn't do the repair. Got my skinny, strong tall grandson, then 13, to get his head and shoulders in the compartment to do the job. I lease him out for vacuflush repairs. Freight and feeding is not included.
As long as you got it apart do the bellows and duckbill valves at same time, you will be glad you did.