Why does the gastric lavage machine only feed but not drain

There is no effective negative pressure inside the machine. Gastric lavage machine is through the negative pressure inside the machine drainage, appear gastric lavage machine only into the water does not drain, because there is no effective negative pressure inside the machine. Gastric lavage machine is a medical equipment, suitable for medical units, emergency centers, such as the rescue of food poisoning, poisoned patients and gastric lavage before surgery.