Yeah, as long as there are no splits or cracks in it it should be ok - perhaps it wasn't seated correctly you'll see that the outside of it is formed to created a seal, might not have been seated properly

Maybe run a smidge of grease around the outside before you put it back in. Check the surfaces that mate with it on the tap - how do they look??
Put the fuel tap back together (still off the tank of course!) and suck (easy Tony) on the diaphragm port on the tap - you should 'feel' the diaphragm opening and closing put you shouldn't be able to draw air through it.
....Now go and sit in a corner for the rest of the day feeling nauseous from the petrol vapor!!
If air is getting through then there's still something wrong. If the diaphragm seems fine and the mating surfaces are ok... well, I'm stumped...
