and the downstream boxes 19 are at the high vacuum level it should be understood that more or less levels of vacuum may be used and more or less boxes for each vacuum level may be provided vacuum is delivered to the sets of vacuum boxes 17 19 through large ducts 21 22 and 23 respectively that run parallel alongside the wire 12 vacuum is developed by large pumps one for each vacuum level that operate continuously when the system 10 is operated the invention contemplates rapid modulation of the vacuum applied to an individual vacuum box 17 19 to draw water from the mat while reducing the tendency of the mat to crack from water removal induced shrinkage vacuum is modulated by a separate valve 26 for each vacuum level diagrammatically represented in fig2 inserted in a line between a duct 21 23 and respective vacuum boxes 17 19 each duct 21 23 serves as both a manifold and due to its volume an accumulator of vacuum storage a valve 26 operates cyclically by rotation developed by a motor 27 that preferably is speed adjustable the illustrated valve 26 has three inlet ports one for each vacuum box 17 a b c or 18 a b c or 19 a b c with which it is associated an outlet of a valve 26 is connected to one of the vacuum ducts 21 23 the valve 26 shown in fig2 is connected to a low level duct 21 and three corresponding vacuum boxes 17 a.