Once you remove the pressure screw, remove the pressure screw handle and flip the pressure screw upside down. Now thread the top of the pressure screw onto the top of the pressure spring piston rod and pull up on the pressure screw nut to pull the pressure spring assembly out of the lubricator body. Depending on the type of lube, and the condition of the lube, in the lubesizer, you may need to heat things up a bit to get the pressure spring assembly out. A hair dryer should provide sufficient heat.
Since you haven't used the lubesizer in a while, I'd recommend using some heat to get all of the old lube out before recharging it with fresh lube.