SERVICE MANUAL is between 524 kPa (76 psi) maximum and 441 kPa (64 psi) minimum.  The contacts will be closed when warning light or electrical buzzer operates. Leakage Test A small vent hole is provided in the cover of the low   pressure   indicator   to   check   the   condition   of   the diaphragm.    Cover  the  vent  hole  with  soap  suds  or  use Leak Detector (SE-2326); if a leak is indicated, it signifies a ruptured diaphragm.  Replace complete assembly. REMOVE 1. The ignition switch should be in the "off" position. 2. Drain the air from the system. 3. Disconnect the electrical connections at the Low Pressure Indicator. 4. Remove the Indicator from fitting. REINSTALL 1. Install indicator in fitting where old indicator was removed. 2. Connect electrical connections. 3. Charge air system and perform Leakage Test. Disassembly  and  reassembly  of  Low  Pressure Indicator is not recommended since it is only serviced as a complete assembly. STOP LIGHT SWITCH DESCRIPTION The   stop   light   switch   (Fig.      6)   is   an   electro- pneumatic switch which operates in conjunction with the brake  valve  and  stop  lights  by  completing  the  electrical circuit when a brake application of 35 kPa (5 psi) or more is made. Fig.  6  Stop Light Switch Both  the  primary  and  secondary  brake  systems are equipped with a stop light switch on a straight truck. If    a    failure    should    occur    in    either    the    primary    or secondary  systems,.    The  system  which  is  functioning properly   will   provide   stop   lights   when   the   brakes   are applied. MAINTENANCE Every three months or every 40,000 km (25,000 miles, check all electrical connections. Every  year  or  160,000  km  (100,000  miles),  the stop light switches should be replaced. SERVICE CHECKS Operating Tests 1. Both    stop    light    switches    must    be    checked independently on a straight truck to be sure both are functioning.  Disconnect one switch. 2. Apply    brake    valve    and    note    that    with    first downward movement of pedal or treadle that the stop lights go on immediately. 3. Release brake valve and note that stop lights go off. 4. If lights fail to go on use a test light at stop light connections.  Test both terminals to determine if an  electrical  supply  is  available  at  switch;  then "bypass" the switch with test light or jumper wire. Lights   should   go   on.      If   not,   a   failure   in   the electrical   circuit   is   the   problem.      However,   if lights work, replace stop light switch. 5. Repeat item 4 to check second light switch. Leakage Test With   brakes   applied   there   should   be   no   air leakage at stop light switch. REMOVE 1. Disconnect  electrical  connections.    Be  sure  to keep    electrical    connections    from    frame,    etc. Tape them. 2. Remove switch from air fitting. INSTALL 1. Install stop light switch in air fitting. 2. Install electrical connections. 3. After   stop   light   switch   is   reinstalled,   perform SERVICE CHECKS. CTS-4079 - CHAPTER U - PRINTED IN UNITED STATES OF AMERICA Page 6