Fluidplay

I found a leak on the brake fluid pressure reg. at the nut on top of the unit where the hard line attaches. I looked up the part on Pelican and noticed an O-ring between the nut and the body. Mine is void of this O-ring, but torqued extremely tight. a) I'm assuming this O-ring went missing at some point in its' life and the PO or shop just tightened it down and it held for awhile. b) If this O-ring lies between the nut and the body of the reg., how tight should it be? Fortunately, I happen to have an additional one on an old M.C. that has this O-ring.
Appreciate the help as always