Yeah, but I think all those bits are for the head which is fine. I removed that and still had the problem. It's a cool pump in that you can disassemble every single piece.
Still, there should be some detail about the piston bit somewhere on the 'net. Normally you just throw some grease on the o-ring and pumpy pumpy improves.
https://www.topeak.com/global/en/products/256-Pump-Replacement-Kit/442-parts-kit
Yours?