To get 0 rebound, you'll need to bleed the shock. I like to have about 1/2 a shaft length of rebound in my shocks. If you have more than that after building them with nothing inside the bladder, hold it sideways, push the shaft in about 1/2 way and loosen the cap until you feel the pressure on the shaft release, then tighten it up again. This should give you about the right rebound. Try and make sure both sides are as close to matched as you can get.