A headbox for a paper machine, in particular a secondary headbox for applying a stream of a coating composition onto a moving wet pulp substrate in a paper machine, comprising - a stock chamber - a stilling chamber - a slice chamber - a valve assembly for controlling the flow of coating composition from the stock chamber into the stilling chamber and for controlling the flow of coating composition from the stilling chamber to the slice chamber wherein the valve assembly comprises a rotatable valve body and a valve case body, said rotatable valve body being located within the valve case body, wherein said rotatable valve body is rotatable into a closed state and into one or more opened states, wherein said rotatable valve body comprises a first and second sealing element (7, 8) and said valve case body comprises a first and second sealing surface (9, 10), wherein in a closed state, a) the first sealing element is in sealing contact with a first sealing surface on the valve case body, thereby forming a first seal which seals the stock chamber from the stilling chamber, and/or b) the second sealing element is in sealing contact with a second sealing surface on the valve case body forming a second seal, which seals the stilling chamber from the slice chamber, wherein in an open state, a) the first sealing element is at a first distance d 1 from the first sealing surface on the valve case body forming a first clearance and thereby fluidly connecting the stock chamber to the stilling chamber and b) the second sealing element is at a second distance d 2 from the second sealing surface on the valve case body forming a second clearance and thereby fluidly connecting the stilling chamber to the slice chamber, and wherein when the rotatable valve body is rotated from an open state to the closed state, the first and second sealing elements (7, 8) are simultaneously moved along a circular path into sealing contact with a first and second sealing surface (9, 10) wherein when the rotatable valve body is rotated from an closed state to the open state, the first and second sealing elements (7, 8) are simultaneously moved away along a circular path C from a first and second sealing surface (9, 10), and wherein in any open state, the first clearance is larger than the second clearance. - a stock chamber - a stilling chamber - a slice chamber - a valve assembly for controlling the flow of coating composition from the stock chamber into the stilling chamber and for controlling the flow of coating composition from the stilling chamber to the slice chamber wherein the valve assembly comprises a rotatable valve body and a valve case body, said rotatable valve body being located within the valve case body, wherein said rotatable valve body is rotatable into a closed state and into one or more opened states, wherein said rotatable valve body comprises a first and second sealing element (7, 8) and said valve case body comprises a first and second sealing surface (9, 10), wherein the valve assembly comprises a rotatable valve body and a valve case body, said rotatable valve body being located within the valve case body, wherein said rotatable valve body is rotatable into a closed state and into one or more opened states, wherein said rotatable valve body comprises a first and second sealing element (7, 8) and said valve case body comprises a first and second sealing surface (9, 10), wherein in a closed state, a) the first sealing element is in sealing contact with a first sealing surface on the valve case body, thereby forming a first seal which seals the stock chamber from the stilling chamber, and/or b) the second sealing element is in sealing contact with a second sealing surface on the valve case body forming a second seal, which seals the stilling chamber from the slice chamber, a) the first sealing element is in sealing contact with a first sealing surface on the valve case body, thereby forming a first seal which seals the stock chamber from the stilling chamber, and/or b) the second sealing element is in sealing contact with a second sealing surface on the valve case body forming a second seal, which seals the stilling chamber from the slice chamber, wherein in an open state, a) the first sealing element is at a first distance d 1 from the first sealing surface on the valve case body forming a first clearance and thereby fluidly connecting the stock chamber to the stilling chamber and b) the second sealing element is at a second distance d 2 from the second sealing surface on the valve case body forming a second clearance and thereby fluidly connecting the stilling chamber to the slice chamber, and a) the first sealing element is at a first distance d 1 from the first sealing surface on the valve case body forming a first clearance and thereby fluidly connecting the stock chamber to the stilling chamber and b) the second sealing element is at a second distance d 2 from the second sealing surface on the valve case body forming a second clearance and thereby fluidly connecting the stilling chamber to the slice chamber, and wherein when the rotatable valve body is rotated from an open state to the closed state, the first and second sealing elements (7, 8) are simultaneously moved along a circular path into sealing contact with a first and second sealing surface (9, 10) wherein when the rotatable valve body is rotated from an closed state to the open state, the first and second sealing elements (7, 8) are simultaneously moved away along a circular path C from a first and second sealing surface (9, 10), and wherein in any open state, the first clearance is larger than the second clearance.