An apparatus for producing an object by sequentially forming thin layers of a construction material one on top of the other responsive to data defining the object comprises a printing head (52) block comprising at least one printing head (52) comprising a manifold through which construction material is dispensed; a first and second reservoir (401,403) connected to ends of the manifold (420), wherein the first reservoir (401) is adapted to receive construction material to be used by the at least one printing head and the second reservoir (403) connected adapted to collect construction material flowing from the first reservoir; and a vacuum pump coupled to one of the first or second reservoirs adapted to maintains a vacuum in the first and second reservoir.