[P05Crossfire]

I have a 2008 Sentra S with 73,000 miles. When I come to a stop with my foot on the brakes and in drive gear the engine idles rough at 600 rpms, but as soon as I release the brake pedal the the engine runs smooth and the rpms go to 750. If I have the A/C on the engine stumbles even worse. Could there be a vacuum leak in the power brakes system that are the cause and how can that be checked? Or any other ideas? Thanks.