An apparatus, comprising: a valve assembly configured to be disposed in a receptacle of an endoscope handle, the receptacle including a gas inlet port, a gas outlet port, a water inlet port, and a water outlet port, and configured to be in fluidic communication with a gas supply system and a water supply system, the valve assembly including a valve stem including a valve inlet port, a valve outlet port, a vent, a first channel defining the valve inlet port and the valve outlet port, a second channel intersecting the first channel and defining the vent, a third channel intersecting the first channel, and a fourth channel fluidically coupled to the third channel, and an adjustable sealing member disposed about the valve stem, the adjustable sealing member aligned with and fluidically coupled to the fourth channel, the valve assembly configured to be transitioned between a first configuration in which the gas inlet port is placed in fluid communication with the valve inlet port such that gas can flow through a first fluid flow path in the valve assembly from the gas supply system, through the valve inlet port, and out the vent, and a second configuration in which the gas inlet port is placed in fluid communication with the gas outlet port when the vent is digitally covered by a user's finger such that gas can flow through a second fluid flow path in the valve assembly from the gas supply system, through the valve inlet port, and out the valve outlet port and such that gas within the valve stem exerts a sealing force on the adjustable sealing member via the fourth channel to provide an airtight seal between the gas ports (252, 254) and the water ports (256, 258). a valve assembly configured to be disposed in a receptacle of an endoscope handle, the receptacle including a gas inlet port, a gas outlet port, a water inlet port, and a water outlet port, and configured to be in fluidic communication with a gas supply system and a water supply system, the valve assembly including a valve stem including a valve inlet port, a valve outlet port, a vent, a first channel defining the valve inlet port and the valve outlet port, a second channel intersecting the first channel and defining the vent, a third channel intersecting the first channel, and a fourth channel fluidically coupled to the third channel, and an adjustable sealing member disposed about the valve stem, the adjustable sealing member aligned with and fluidically coupled to the fourth channel, the valve assembly configured to be transitioned between a first configuration in which the gas inlet port is placed in fluid communication with the valve inlet port such that gas can flow through a first fluid flow path in the valve assembly from the gas supply system, through the valve inlet port, and out the vent, and a second configuration in which the gas inlet port is placed in fluid communication with the gas outlet port when the vent is digitally covered by a user's finger such that gas can flow through a second fluid flow path in the valve assembly from the gas supply system, through the valve inlet port, and out the valve outlet port and such that gas within the valve stem exerts a sealing force on the adjustable sealing member via the fourth channel to provide an airtight seal between the gas ports (252, 254) and the water ports (256, 258).