A valve apparatus device provides surgical instruments with sealable access to an inlet port of an endoscope instrument channel. The valve has a hollow body with a distal end that releasably attaches to the inlet port, and a flexible diaphragm seal dividing the body into a proximal chamber and a distal chamber. An instrument opening extends through the diaphragm seal for the passage of instruments therethrough, and connects the chambers together. The diaphragm seal is configured to seal with a surgical instrument inserted into the instrument opening. At least one fluid transfer member is provided to fluidly interconnect the proximal chamber to the distal chamber when the surgical instrument is sealed within the instrument opening.