When using a Buchner funnel with a rubber cone, the seal is formed between the top of the cone and the tapered part of the underside of the funnel, so it is best to gently push down on the top of the funnel when filtering. The important part is to ensure you have the correct size of rubber cone to fit snugly in the neck of the filter flask. The rubber cone does not need to fit tightly onto the Buchner funnel before using.