Chances fell to either side with Phillips forcing a good save out of Lonergan. Bent was a handful for the North End defence as was Cameron Jerome when he came on.
Wallace who started on the left and switched to the right was at the hub of all North End's attacks. He created the goal with a superb cross. Parkin was free in the box and blasted past the Birmingham keeper with the game entering stoppage time.
Parkin should have scored earlier when put through the ball got stuck at the big man's feet before the keeper raced out and claimed the ball.
Preston now sit in the top six and will look to consolidate over the fextive period.