Hello,
I had attempted to rebuild my 85 300D pump and all was going well until I overtightened the 10mm nut holding on the piston. I broke the threaded portion that holds the center of the piston diaphram and decided to just throw down 220 and get a new pump.
I have since rebuilt 2 other pumps with success, and they are working well still. The one-way valves often will go bad perhaps your piston sleeve won't need to be replaced.
Good luck, Adam B