Your search for "Fleck 2510 Service Kits" revealed the following:

Seal, Molded Outlet, TwinFlo 8500 - 19005 *OBSOLETE*
As of 01/01/18 - this item is no longer in production, and is no longer
available from the manufacturer. Please call us at 888-426-5001 for...
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Parts & Accessories
Connector Assembly, 3/4" MNPT Noryl w/O-Ring (2-Pack) - 42414-01
$9.25
Connector Assembly, 3/4" MNPT Noryl w/O-Ring (2-Pack)
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Availability: Out of Stock
Gear, TwinFlo 8500 - 19062

Gear, TwinFlo 8500

Pinion Drive, TwinFlo 8500 - 18796

Pinion Drive, TwinFlo 8500