The standard 60090 Piston Assembly can be used for most residential applications.
The "High Flow" Piston is normally used on Fleck 2750 & 2900 (Upper Piston), 2 Piece Casting Valves, when the backwash flow rate is higher than 7 GPM, or if the Service Flow Rate is higher that 12 GPM.
>>>If you have a 1 Piece Casting, then you should use Fleck Part # 61540 & Seal Kit, that has a longer Shaft.
This Part can be located on page 16, of the Fleck 2900 Service Manual. Free Fleck Service Manuals, can be located on the Index, on the Left Side of our website.
2700, 2750, 2900 Main Piston Assembly for Higher Flow rates common for applications that use the 2700, 2750, or 2900 Fleck Model Valves.
Part Number for Assembly is 60090-HF, this part comes complete with seal o-ring end plug PN 40078, old part number 10234, and quad o-ring ( 10209 ).