Never thought about the rubi, but as Eric said I have also extended alot of PoPo's. If you look at the overflow cap there is a small hole in it. This alows it to vent.
Just think of the stuff you ride in. As the cooling system sucks fluid from the reservoir to maintain the cooling system, guess what else it's sucking in through the weep hole. The muddy water that you are riding in.
What we have done is drill out the weep hole and add a double ended 1/4" king nipple with a hose and extended it to the pod. The hole only needs to be big enough that you have to force the fitting in the cap. No RTV is needed to seal the fitting up. The cap is plastic and will form around it. Just remember to point the hose end away from you. If for some reason it decide to push hot fluid out, it want blow it on you.