Removing Silverado power brake booster unit

I am having to replace the power brake booster unit and master cylinder in my 1996 pickup. I've got the new unit installed and am having trouble with the little clip that goes on the shaft of the break pedal assy. It is the one that goes on after the plastic brake light assy snaps on. There is first, a small ? spacer ? with pressure springs formed into both sides and it goes next to the plastic, then this ? collar ? that snaps around the shaft. My question is just what function does that ? spacer ? perform? It seems to just spin around on the shaft and do nothing.