Smart man, I was going to suggest breaking the seal on the cap. It will be fine as long as you don't blow too much water out, and that probably won't happen. I hope that the O-ring is the problem. If not, I would try some Alumaseal. It's powered aluminum sold at any parts store and it works great. But brace yourself, it'll set you back about $2. I've tried and have seen others try different brands of sealers without much luck, or only to have it leak again months or weeks later. Alumaseal will stay in the system and patch future leaks as well.

Good luck!!