Common o-rings that need replacing for the valve to tank connections are 19936 ( # 7 in diagram below )
Seal_Base_Seal_Fleck_2510_PN 19936
and
18303 ( # 14 in diagram below )
Oring_Tank_Adapter_5000_2510_9100_PN_18303

* Also, check the condition of the Slip Ring part # 19197 ( # 13 in diagram below ) for Base of 2510 Adapter


For other parts for the Fleck 2510 valve, see here: Fleck_2510_Parts
Link to the Fleck 2510 Manual:
Service Manual Fleck 2510

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And for LEAKS out the DRAIN HOSE: Fleck valves use pistons and internal seals to control water flow through the control valve head.
When water continuously flows out the drain hose, and the unit has finished the regeneration process ( IN SERVICE position ),
it is usually a "bad" or "worn" SEAL, and replacing the Seals & Spacers is required.
It is also normal to change the Main Piston at the same time,
as age will effect this part over the years,
and as long as you have the valve apart,
it makes sense to put a new piston in at the same time.