| http://www.w3.org/ns/prov#value | - Sealing ring 28 (see FIG. 1) is secured in channel 30 (see FIG. 4) in the outer periphery of valve unit 24 to provide an airtight seal with the inner surface of chamber 12, and in addition a pre-filter 32 is seated in the lower end of primary valve unit 24 to filter or strain out in particular larger particles or contaminants that would otherwise be able to enter unit 24 and clog check valve 26.
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