A yarn feeder includes a storage drum, and an unwinding brake which has a storage drum counter-braking surface continuous in the peripheral direction and coaxial to the axis of the storage drum. The unwinding brake further has a braking component supported on the counter-braking surface axially counter to the direction of yarn unwinding so as to produce spring action and having a coaxial braking surface also continuous in the peripheral direction. The yarn feeder also has a mounting for the braking component, and a pneumatic brake opening device which has a pressure chamber with a pneumatically loadable, axially movable boundary wall. The boundary wall is connected to the braking component so as to transmit movement at least in the direction of movement there of counter to the action of the spring. When the boundary wall is pressurised by excess pressure or a vacuum, the braking component can be moved axially into a threading position in which the braking surface is lifted from the counter braking surface.