Abstract: A regulator (20) for an ink-jet pen (24) that has a print head (30) for expelling ink from a fluid volume includes a seat (36) mounted to a pen body (26) and having a port (42) formed through it. A valve element (38) is mounted adjacent to the seat for movement relative to the port (42). Magnetic attraction is employed for urging the seat (36) and valve element (38) together to thereby close the port (42) and permit underpressure to develop in the reservoir (22). The valve element (38) and seat (36) are configured and arranged so that when the underpressure within the reservoir (22) rises above the level that may cause failure of the ink-jet print head (30) the valve element (38) moves away from the seat (36) to permit air to enter the reservoir (22), thereby reducing the underpressure to an operable level.