It's actually pretty simple. You'll get that action you're describing sometimes. For your situation, it's probably not a huge deal.
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Here's how I build shocks - Fill the body with oil to the bottom of the threads. Insert the cartridge and tighten it down by hand. Make sure you wrap a rag around it - it will get messy. You may have to untighten it a little to get the piston to move. Tilt the shock body in your hands at a 45 degree angle. Push the shaft and piston in slowly and push the excess oil out around the cartridge. Once you get the piston pushed all the way in, use your RPM Shock Wrenches to snug down the cartridge. The shaft and piston should have a little reboud - meaning the shock shaft should push out a centimeter or two. That's it! Put the spring on and off you go. Good luck. If you have any other concerns, Private Message me.