A 3D boot has less elasticity on the sides than a rubber boot. In that case, I'd mount the boot to the ambient cap before you screw it down. It'll be less easy to slip a syringe under the edges with a 3D plastic boot. Then, when you screw down the cap and the grease starts coming out of the holes, massage the boot to work the air out as you screw it down. The air will follow the path of least resistance around the edge of the boot.
Pressurize as before, massage flat, wipe and depressurize.
Pressurize as before, massage flat, wipe and depressurize.