A valve system regulates the air pressure in the space(s) between a residual limb, or liner-covered limb, and a hard socket of an external prosthesis, and may include a manually-controlled air outlet and inlet valve on a distal region of a hard socket, and/or an automatic one-way outlet valve. The manually-controlled valve is opened and closed by partial rotation/twisting of a handle portion, which creates slight separation of the handle and base portions through which may pass air. The handle and base are prevented from becoming entirely separated during normal use by a snap-fit of the base onto the handle that retains the ability of the handle and base to rotate relative to each other by means of a ramp system, and a stop(s) that limit(s) the amount of relative rotation of the handle and base portions of the valve.