+1 to that.Usually caused by the inner tube moving around the inside by the tyre under braking, also caused by a rough burr on the inside of the valve hole on the rim.
Check the valve hole on the rim for rough edges, if still happening some talc inside the tyre stops the tyre gripping the inner tube and dragging it round while braking.
Talc (or other powder of your choice) should fix it.
And don't leave the valve lock ring done up really tight once the tube is pumped up.