A liquid dispensing apparatus is provided which has a syringe for storing and dispensing a liquid, and a connector passage connected to the syringe. A positive pressure passage is connected to an air source for supplying positive pressure to the syringe, and there is a pressure valve located along the positive pressure passage that is operative to control the flow of air along the positive pressure passage. A vacuum passage is connected to a vacuum generator for supplying vacuum pressure to the syringe, and there is a vacuum valve located along the vacuum passage that is operative to control the flow of air along the vacuum passage. The positive pressure passage and vacuum passage are connected to the connector passage. A pressure sensor is also provided to measure the pressure in the connector passage.