A master frame assembly for use in a screen printing press carries an upper seal means to seal directly to a screen printing frame to provide an enclosed sealed chamber within the screen frame to contain solvents associated with the ink within the sealed chamber. The preferred upper seal means includes a traveling cover sheet which is pressed directly into sealing engagement with the underlying screen frame about all four sides of the screen frame. Mounted on the master frame are side seal bars for pressing the traveling marginal edges of the sheet against the underlying sides of the screen frame. These side seal bars are adjustably and movably mounted on the master frame so that they may press the traveling cover sheet against the sides of each of several different widths of screen frames.