A separating tank assembly enables a dental vacuum separating tank to be drained without using a pump and without turning off or isolating the vacuum source from the tank. A drain tank is provided in fluid communication with the separating tank, and a control valve controls a pressure differential between the drain tank and the fluid tank. When the separating tank is filling, the drain tank and separating tank pressures are equalized, and the tanks are in fluid communication. When the fluid in the drain tank reaches a predetermined level, the solenoid opens the drain tank to atmosphere, blocking fluid communication between the separating tank and the drain tank and opening fluid communication between the drain tank and a sewer drain. After the fluid is drained, the solenoid reopens fluid communication between the separating tank and the drain tank, and the pressures in the respective tanks are equalized.