Thanks, but this isn't for the type of system i have. It appears my type is often called the "early" type and is only used before '97.
I did find a random o-ring lying around that fit pretty well but i'm not sure how long it'll last. The original one was brittle and hard as a rock but i was able to get some measurements.
Old seal (approximate)
Thickness (OD to ID) 1,50mm
Reservoir connector's groove for the seal
The diameter of the hole in the pump where the reservoir connects is 13,50mm.
Here's a picture of the system with the o-ring in question highlighted.