[szyzygy]

I put the battery back in my car today because the weather seems to be warming up here in VT.
I noticed that one of the little plastic cap/plugs on my master cylinder/booster is missing. It's the bottom one.
It was there in December (I have pics to prove it) when I stopped driving it and had it shipped from CA to VT where it has been sitting for the last 2 months.
I tried looking around to see what this plug does but no luck.

Can one of you guys let me know 1) what that bottom cap/plug does and 2) is the car usable without it?

Thanks