The present invention relates to a mould assembly (100) for manufacturing a wind turbine blade shell part, and to a method of manufacturing a wind turbine blade shell part using the mould assembly (100). The mould assembly (100) comprises a lowering device (85), which is adapted to carry and lower a root end insert onto the moulding surface of the mould, the lowering device (85) being attached to the mould and comprising a frame (86) for carrying the root end insert. Two synchronized hydraulic cylinders (91, 92) are used as driving means for lowering the frame, each hydraulic cylinder comprising a piston chamber and a rod chamber, wherein the piston chamber and the rod chamber of each cylinder are connected to each other via a respective valve assembly (110) comprising a fluid line (96) and a valve (97).