SERVICE MANUAL will  not  shut  off,  this  indicates  an  open  drain  cock  or  a failure in the brake system. It   is   not   compulsory   but   it   is   advisable   that vehicles be inspected to be sure that the air gauges are properly   connected.      The   primary   system   should   be connected to the green needle and secondary system to red needle.  This can easily be checked by charging the air  system,  bleeding  off  the  primary  system  (rear  brake reservoir)   and   the   green   needle   should   drop.      If   the green  needle  does  not  drop  the  air  lines  on  the  gauge should  be  changed.    All  chassis  would  be  assured  that they are connected in the same manner and identification of both systems will be uniform. SERVICE CHECKS 1. Check the air gauge for accuracy.  The simplest way   do   do   this   is   to   compare   the   pressures registered by the gauge over its normal pressure range   with   the   pressure   registered   by   a   test gauge known to be accurate. 2. A    gauge    which    loses    its    accuracy    must    be replaced.    The  continued  use  of  a  dash  gauge showing an error of more than 35 kPa (5 psi) is not recommended. LOW PRESSURE INDICATOR;(LP-3 TYPE) DESCRIPTION The  low  pressure  indicator  (Fig.    4)  is  a  safety device designed to give an automatic warning whenever the  air  pressure  in  the  primary  or  secondary  air  brake system    is    below    approximately    483    kPa    (70    psi). Operating   as   an   air-controlled   switch   of   an   electrical circuit, the low pressure indicator automatically sounds a buzzer   when   the   air   pressure   drops   too   low.      The warning will be both visible (light) and audible (buzzer). The nominal pressure setting of 483 kPa (70 psi) is subject to a tolerance of plus or minus 41 kPa (6 psi) so that the actual operating pressure of the low pressure indicator  may  vary  between  524  kPa  (76  psi)  maximum to 441 kPa (64 psi) minimum. Fig.  4  Exterior View of Low Pressure Indicator OPERATION (Fig.  5) To describe the  operation,  we  shall  assume  the Low   Pressure   Indicator   is   set   for   483   kPa   (70   psi). Setting  of  indicator  is  marked  on  a  label  on  valve  body. When    air    pressure    at    supply    port    and    under    the diaphragm is above 483 kPa (70 psi), electrical contacts remain  open  because  the  force  exerted  by  air  pressure underneath  the  diaphragm  overcomes  force  exerted  by the spring above the diaphragm. When  air  pressure  below  the  diaphragm   drops   below 483   kPa   (70   psi),   the   spring   exerts   a   force   which   is greater than the force exerted by the air pressure below the  diaphragm.    This  causes  the  piston  to  move  and allow the electrical contacts to close.  This completes or closes electrical circuit to warning device, warning driver of low air pressure in the system. Fig.  5  Cross Sectional View of Low Pressure Indicator 1 Cover 4 Piston 2 Spring 5 Body 3 Contact Points 6 O-Ring Diaphragm MAINTENANCE Every  three  months  or  after  40,000  km  (25,000 miles), check electrical connections. Every    year    or    160,000    km    (100,000    miles), perform  SERVICE  CHECKS.    If  diaphragm  is  ruptured, replace complete assembly. SERVICE CHECKS Operating Test Operation  of  the  low  pressure  indicator  may  be checked  with  ignition  switch  on,  then  by  reducing  the reservoir pressure and being sure that the contacts close when the reservoir pressure CTS-4079 - CHAPTER II PRINTED IN UNITED STATES OF AMERICA Page 5