A valve mechanism comprising a valve body having an inlet port and an outlet port and a passageway extending therebetween, and a flow blocking element provided in the passageway to block flow of a substance between said inlet and outlet ports, and means for breaking said flow blocking element to thereby permit said substance to flow in a flow direction from said inlet port to said outlet port and further comprising a filter provided in said passageway between said flow blocking element and said outlet port, to filter said substance before it flows out of said outlet port, said filter separating said passageway into a filtered region and an unfiltered region, wherein said filter has a first surface that is in flow communication with said unfiltered region, said first surface having an outer circumference, and wherein said filter is positioned within said passageway so that a first section of passageway wall extends circumferentially around the outer circumference of said filter, and so that a space is provided between at least a part of said outer circumference of the first surface and the section of wall extending circumferentially around it, said filter comprising a bowl shaped mesh, said first surface extending from a first, open end to an opposing second end, in a direction opposite to said flow direction, and wherein said filter is positioned in said passageway so as to provide said space between at least a part of said outer circumference of said first surface, to thereby allow said substance to flow across said first surface of said filter from said second end to said first end, and wherein said passageway further comprises a cavity formed in a section of said unfiltered region of the passageway, and characterised by said passageway including: a first gradual increase in circumference upstream of an entrance to the cavity; and a second, abrupt increase in circumference of the entrance of the cavity. a valve body having an inlet port and an outlet port and a passageway extending therebetween, and a flow blocking element provided in the passageway to block flow of a substance between said inlet and outlet ports, and means for breaking said flow blocking element to thereby permit said substance to flow in a flow direction from said inlet port to said outlet port and further comprising a filter provided in said passageway between said flow blocking element and said outlet port, to filter said substance before it flows out of said outlet port, said filter separating said passageway into a filtered region and an unfiltered region, wherein said filter has a first surface that is in flow communication with said unfiltered region, said first surface having an outer circumference, and wherein said filter is positioned within said passageway so that a first section of passageway wall extends circumferentially around the outer circumference of said filter, and so that a space is provided between at least a part of said outer circumference of the first surface and the section of wall extending circumferentially around it, said filter comprising a bowl shaped mesh, said first surface extending from a first, open end to an opposing second end, in a direction opposite to said flow direction, and wherein said filter is positioned in said passageway so as to provide said space between at least a part of said outer circumference of said first surface, to thereby allow said substance to flow across said first surface of said filter from said second end to said first end, and wherein said passageway further comprises a cavity formed in a section of said unfiltered region of the passageway, and characterised by said passageway including: a first gradual increase in circumference upstream of an entrance to the cavity; and a second, abrupt increase in circumference of the entrance of the cavity.