SERVICE MANUAL suds.  Leakage in excess of a 77 mm (3") soap bubble in 3 seconds is not permissible. 2. Leakage   is   caused   by   dirty   or   scored   key   or body.      Leakage   due   to   dirt   is   corrected   by cleaning parts and applying a thin coating of Item 1 in LUBRICANT SPECIFICATIONS on the key. Leakage due to a scored key or body cannot be repaired and the drain cock must be replaced. SAFETY VALVE DESCRIPIION The purpose of the safety valve is to protect the air brake system  against  excessive  air  pressure.    Should  the  air pressure in the air brake system rise above the setting of the  safety  valve  at  934  kPa  (150  psi),  the  valve  opens and  permits  pressure  above  934  kPa  (150  psi)  to  be exhausted.  It is located on the supply reservoir. The  safety  valve  is  a  piston  type  valve  (Fig.    2).    The piston is equipped with an O-ring type seal which seats in the body of the valve. Fig.  2  Sectional View of Piston Type Safety Valve 1 Piston and 3 Spring Shaft Assembly 4 Nut 2 Body 5 Stake Lock MAINTENANC! Once  each  year  or  every  160,000  km  (100,000 miles),    the    safety    valve    should    be    removed    and thoroughly cleaned. SERVICE CHECK Leakage Test Leakage at the piston in the body should not exceed a 77 mm  (3  in.)  soap  bubble  in  3  seconds.    If  air  leakage  is excessive  the  valve  must  be  replaced  since  it  is  only serviced as a complete component. The safety valve is preset to "blow off" at approximately 934   kPa   (150   psi).      Since   the   safety   valve   must   be removed to perform any adjustments, it is suggested that the   valve   be   replaced   when   any   defect   is   detected. CAUTION When replacing the safety valve be sure to drain all air from the supply reservoir, to prevent bodily injury when the valve is removed.    Draining  of  the  primary  and secondary    reservoirs    is    not    required since    they    are    protected    by    check valves. AIR PRESSURE GAUGE DESCRIPTION The   purpose   of   the   air   pressure   gauge   is   to register  the  amount  of  reservoir  air  pressure  in  the  air system.      While   air   pressure   gauges   of   this   type   are commercially   accurate,   they   must   never   be   confused with or substituted for test air gauges, which are intended primarily  for  accurately  checking  air  pressure  in  the  air brake system. The   air   pressure   gauge   may   be   either   two gauges    or    a    single    gauge    with    two    indicators    as illustrated (Fig.  3). An   air   pressure   gauge   is   located   in   both   the primary and secondary brake systems so that the actual air pressure in both systems is indicated to the operator of the vehicle. Fig.  3  Air Pressure Gauge Only test gauges known to be accurate are to be used    for    checking    brake    valve    delivery    pressures, governor pressure settings and other tests.  Test gauges differ  from  ordinary  dash  gauges  in  respect  to  material and  workmanship.    Due  to  these  differences  they  are more   accurate   over   entire   range   and   maintain   their accuracy over longer periods. After initial starting of engine, if air gauge shows that one circuit  has  no  air  pressure  nor  a  noticeable  increase  in air pressure within a reasonable length of time (30 to 60 seconds) and the low pressure switch and low pressure light CTS-4079 - CHAPTER II - PRINTED IN UNITED STATES OF AMERICA Page 4