Stumpy

so what is the trick to getting the slide rubber part to stay in place when putting it back together

Wolverine

The diaphragm? No tricks that I'm aware. Did you soak it in carb cleaner or another liquid? That can distort and mis-shape. If no, just keep seating the edge till it sits correct. Push it in around the seats edge till it lays in there without the need to hold any part of it.

Hangfly