indewildemanfan said:
I was under the impression that you didn't need to add any c02 until about halfway through the keg? I do have a c02 cap but what do you mean my beer will only last a week without C02? Do you mean it will be flat within a week?
Unfortunately he doesn't just mean it'll be flat in a week. It'll go off and oxidise in a week because opening the cap has let air in. Unless you're having a party I'd be inclined to prime with new sugar, bring it into the warm and (once it gets going) release some pressure from the pressure relief valve (if there is one) or by unscrewing very slightly. By doing this a few time you should be able to purge the headspace above the beer of the oxygen that got in when you unscrewed.
How did you prime the barrel first time round? It can't have worked properly if you ran out of pressure after half a pint.