A biopsy device comprises a body, a needle, and a cutter operable to sever tissue protruding through a lateral aperture of the needle. A vacuum pump is in fluid communication with the cutter. A fluid pump is in fluid communication with the needle. A fluid pump actuation assembly is operable to actuate the fluid pump to deliver a bolus of fluid to the needle based on movement of the cutter. A valve assembly responsive to cutter movement is operable to selectively couple the needle with the fluid pump, couple the needle with atmospheric air, or seal the needle relative to the fluid pump and atmospheric air. The fluid from the fluid pump and atmospheric air cooperate with the vacuum to urge severed tissue samples proximally through the cutter. Components operable to actuate the cutter and the fluid pump include coaxial lead screws having different pitch diameters.