power steering fluid reservoir missing lock nut/clip

[sleepstar]

soooo tomorrow i was set to flush out the PS fluid and put in a new filter since i know it hasn't been done in at least the last 5 years. i took the cap off this evening and the fluid is nasty brown. but i dont have any of the lock nuts/clips that are in the DIYs on this sight. it's just a loose spring on the central rod. what is the purpose of the nut that compresses the spring? should i get one ASAP before i do the fluid flush??