opposite sides of the body the pivot pin 72 extends through bearing holes in the flanges 82 a vibration damping tension spring 83 see fig4 is stretched between one lever body flange 82 and an ear 42a formed integrally with the support plate 42 the spring 83 maintains the lever body 80 biased toward a positon where the screw 74 maintains light contact with the bellows head when the vehicle is below the 4000 ft elevation the system 10 is calibrated in an atmosphere having a predetermined pressure for example standard atmospheric air pressure at 4 000 feet above sea level when the system 10 has been placed in such an atmosphere the bearing screw 74 is turned by a screwdriver inserted through a small calibration opening 84 in the cover member 44 adjacent the screw the bearing screw is turned until the output pressure from the regulator valve assembly 26 is just sufficient to actuate the control valve 30 the access opening 84 is then covered by an adhesive label or tape to seal the access opening closed against the possibility of atmospheric airborne particles ambient the cover entering the case 32 the regulator valve assembly 26 is a force responsive pressure balanced type pressure regulator which communicates with a source of operating pressure and with ambient atmospheric air to produce an output pressure level which varies between that of the source pressure and atmospheric air pressure the degree of force input to.