When building the Tamiya shocks, I have found it easiest this way.
Remove air bubles with piston at top
Pull piston down (to make sure no more bubbles)
Fill to level with top of shock with piston midway to 2/3 to top
Then set the bladder on top, and press it in a little with a screwdriver.
Then add the oring or sponge to suite track
Then add the top plastic to the top
Gently set the top cap over the plastic snap
Press firmly on the plastic while screwing cap down.
This gives me shocks that last a LONG time
If the piston is in 2/3 when you assemble. The shaft moves the same from top as it does from bottom (same rebound). If you build it with the piston in middle of stroke then you have more compression then rebound.
These are my findings, but should make it easy to duplicate the builds...