Thanks coralman I meant Viton, I was checking out o-rings on the mcmastercarr website.
These are the measurements I got off the o-ring that came with my new reservoir:
Outer diameter - 0.760"
Inner diameter - 0.550"
Width - 0.110"
I measured the width with a screw type micrometer because my digital caliper kept compressing the rubber to much. It was easier to feel the rubber with the micrometer. The caliper was giving me 0.101" for what it's worth.
I measured the plastic nipple on the reservoir and it was 0.592". This means that the radial stretch is 7.64% which is actually beyond the recommended 1%-5%.
My guess is that this is actually a metric o-ring with an inner diameter of 14 mm (0.551") and a width of 3 mm (0.118"). The plastic nipple is right at 15 mm (0.590"). Mcmastercarr sells a pack of 14 viton o-rings in this size for $7.
I might get around to swapping the new reservoir over this Sunday and I'll measure the bore on the pump and the amount of distortion in my old reservoir to see what o-rings are available for those of us with old reservoirs.
At first look it seems that a 1/8" width, 5/8" inner diameter o-ring might work for an old reservoir. This assumes that the reservoir has shrunk until the factory o-ring has no stretch over the plastic nipple. With this o-ring, assuming the pump bore is 19 mm, you get 3% stretch radially, and 25% compression. That's right about perfect but I still need to double check how much the old reservoir has shrunk and what the bore diameter is. My old reservoir nipple as visibly cone shaped from shrinking under the o-ring.