the piston 9 in order to prevent blowby around the filter 11 an annular seal 18 around the piston 9 makes a sealing contact with the tube wall 12 to prevent blowby around the piston 9 and to force displaced serum 4 to pass through the aperture 17 during the separation phase of centrifugation the sedimentation velocity of the serum preparator 1 is restrained by a brake 6 or combination of brakes braking the sedimentation velocity causes several effects the braking means slows the sedimentation of the piston 9 so that the piston 9 overtakes fewer slowly sedimenting particulates including slowly sedimenting uncoagulated cells the braking means also reduces blow by around the filter 11 by reducing the pressure difference across the piston 9 and the filter 11 at a reduced pressure less serum 4 will escape filtration by passing around the piston 9 and filter 11 braking the sedimentation velocity of the serum preparator 1 reduces the rate of filtration and thereby reduces the possibility of filter overload braking the sedimentation velocity of the serum preparator 1 reduces the momentum of the the device and prevents the momentum from causing the device to overshoot its isopycnic equilibrium position in the first embodiment of the braking means a frictional brake 6 reduces the sedimentation velocity of the piston 9 by exerting a frictional drag between the serum preparator 1 and the.