rear mastercylinder leak

Hello i have a small leak between the plastic and aluminum on the master cylinder on the rear. It is sealed up by an o-ring which i replaced with one from the hardware store and now its worse. Is there a sealer i could put on that o-ring. I cant seem to locate one on-line, any ever have this problem.

I had the same problem on my 650 when I was renewing the master cylinder, except it was the front master cylinder that I needed the o-ring for.
I'm sure you already know that there are certain parts on our bikes that can be dismantled, but some of their individual parts (the o-ring that sits between the reservoir cup and master cylinder body for example) are not considered a "serviceable part" from Kawi = unobtainium.

The o-ring you obtained from the hardware store as a replacement might not have been designed to withstand the brake fluid, and could be the reason it is leaking even worse now. Once you "broke the seal" on the mating surfaces of the old o-ring, then the only thing to reseal those surfaces will be a new (and preferably correct diameter) chemical resistant designed o-ring.

I took my reservoir cup and master cylinder body down to my local Pep Boys Automotive Super Center to match an o-ring up for it.
Fortunately I was able to find a chemically resistant o-ring, and one that was only slightly larger in O.D. than the original. Once I tightened the two bolts within the reservoir cup to the body, it compressed that o-ring perfectly.
Over two years and roughly 20K miles later and it's still leak free!