re-assembling a Ruger 22/45 Lite question

[Deepriver]

Anybody own one of these that can help me? I'm trying to put this thing back together and having an issue. See if I can explain. When putting the mainspring back in, the round 'pin' that sticks out of the top of the pistol (right behind the sights) about 1/8" wants to slide back down when I try to collapse the assembly back together. Does this make sense? I watched a you tube video and of course it looked straightforward when he did it, but mine is not behaving the same...Thanks.

Never mind -figured it out!

 

[NKD]

edit:
Ahh just refreshed and saw you figured it. My bad. Have fun.