Abstract: A pressure control device installed at the bottom of the reservoir of an ink-jet pen. The device has a tubular boss whose upper end has an arc surface and that a sphere sits on the arc surface. Ideally, the sphere makes a line contact with the arc surface of the boss. The device also has a spring with one end mounted to the bottom of the reservoir. The spring has two portions together. The first portion of the spring presses tightly against the sphere while the second portion touches a pressure plate inside the reservoir. The reservoir further contains an expandable bag. When the bag inside the reservoir expands, the pressure plate attached to the bag will push the second portion of the spring such that its first portion will move away from the sphere. Due to the presence of a back pressure within the reservoir, the sphere will be afloat briefly permitting ambient air to enter the reservoir. Consequently, back pressure within the reservoir is regulated.