Patient airway baseline pressure levels are controlled in a ventilator having a pneumatically controlled exhalation valve without an external supply of pilot pressure, by supplying an internal source of controllable pilot pressure for operating the exhalation valve. The pressure of pressurized air supplied by an internal pump can be electrically controlled by pulse width modulation of a variable pulse duration signal. A second source of pressurized air for pilot pressure is provided, and in one embodiment is a reservoir provided for receiving pressurized air from the pump and from the exhalation circuit. A valve is connected to the pump means and the reservoir for receiving pressurized air from the pump in a first position, and for receiving pressurized air from the reservoir in a second position. When the valve means is in the second position, pressurized air is delivered at a pressure which is the greater of the pressurized air supplied by the pump means or the gas flow from the exhalation circuit.