Piston and Seals KIT- "Genuine" Fleck 2510 PN 61670-00 includes a PLASTIC ( not brass ) Main Piston Assembly ( same size as the P.N. 60090 ) used in Fleck 1500, 2500, 2510, 2700. NOTE: MacClean Kit # 60121-C uses larger "end spacer" ( re-use your old one ) instead of the small white end spacer ( 10757 ) in this Kit ( 60121 ).
This kit also includes the Seals & Spacers for rebuilding the 2500 or 2510 valve.
The Seals are made of Silicone and not rubber ( like the Seals in Kit 60121 ).
Same size, but different material.
Both versions ( 60090 & 60121 or the KIT 61670-00 ) are Genuine Fleck parts ( from Pentair Water ).
Warning.... some Service Manuals list this as 61670-01 ( NHWBP version that cost about 4X as much ).
And the manuals list NHWBP as PN 61670-02 ( which does not exist in Fleck / Pentair parts catalog ).
The problem is a "typo" in some 2510 service manuals.
NOTE: If the Valve is Leaking from the Front of the Main Piston, you Must Replace the Main Piston Assembly to stop this type of EXTERNAL Leak.
The Seals and Spacers need replacing when you have "internal" leaking of the Valve.
This would be constant flow out the drain line, or failure to create suction ( after injector and brine valve check out okay ), or hard water getting into the house when everything else appears to be working fine.
NOTE: It is difficult sometimes to get all the old seals and spacers out, but a stiff piece of wire and basic hand tools will do the job.
Replacing the Seals is almost impossible to do without the special "stuffer" tool ( PN 11098 ).
The stuffer tool includes directions for it's use. And can be rented ( 1/2 price ) for 30 days.
Directions can be found on page 29 and 30 of this Fleck 2510 Manual https://www.nelsencorp.com/customer/docs/SKUDocs/2510_Fleck%20Service%20Manual.pdf
NOTE: Does NOT fit General Ionics valves.