[Mackie]

Just like your house in the summer you get better "draft" with 2 windows open. Just the vented push cap on the fill tube isn't enough. A `68 327 should have a PCV fitting on the drivers side of the dist hole and a hose to the carb base. Earlier small blocks had a road draft tube that came out here and went down the back of the block.
The P stands for positive where the engine vacuum ensures ventilation, not just the breeze passing across the bottom of the road draft tube.