SERVICE MANUAL RESERVOIR DESCRIPTION The air reservoir (air tank) function is to provide a volume of compressed air used in braking the vehicle. There are at least three reservoirs on trucks with the FMVSS 121 brake system.  These tanks are supply reservoir  and  secondary  and  primary  reservoirs.    The primary reservoir is the air source for the rear brakes and the front brakes are  supplied  by  the  secondary  air  tank. In  some  instances,  the  supply  reservoir  and  secondary reservoir  may  be  in  the  same  tank   with   a   separation inside the tank (Fig.  1). Another  function  of  a  reservoir  is  to  provide  a place  where  the  air,  heated  during  compression,  may cool and cause the oil and water vapors to condense. The  combined  volume  of  all  service  reservoirs and supply reservoirs are 12 times the combined volume of  all  service  brake  chambers  at  maximum  travel  of  the pistons  or  diaphragms.    The  size   of   air   tanks   should never be altered without IH Engineering approval. The   reservoirs   should   be   completely   drained daily.  If an automatic drain device is used, as shown in Fig.        1,    the    automatic    drain    should    be    checked periodically   to   determine   if   it   is   functioning   properly. When  manually  draining  tanks,  satisfactory  draining  is only accomplished by leaving the drain cocks open after all air has escaped and all drainage has stopped. Reservoirs  are  tested  against  a  2068  kPa  (300 psi) and treated on the inside with a rust preventive. SERVICE CHECKS Leakage Tests With  the  air  brake  system  charged,  use  Leak Detector   Tester   (SE-2326)   to   check   for   leakage   on outside  surfaces  of  reservoirs.    If  any  leakage  is  found, replace the reservoir. Inspection Inspect  inside  and  outside  surfaces  for  damage or corrosion.  A small flashlight is helpful when inspecting the  interior.    If  damage  or  corrosion  is  found  that  would weaken the reservoir, replace the reservoir. Moisture    taken    in    with    the    air    through    the compressor  inlet  valve  collects  in  the  reservoirs and  necessitates  draining  the  reservoirs  daily  in cold weather and every week in   warm   weather   by   opening   the   drain   cock located  on  the  bottom.    Be  sure  to   close   the drain cocks after all moisture has been removed. MAINTINANCE Drain air reservoirs regularly as required.  Local conditions    govern    frequency.        In    dry    climates,    for example, once a month may be sufficient, while in humid areas it may be necessary to drain reservoirs daily. When draining the air reservoirs, open the drain cock and let the air bleed off.  Be sure to leave the cock open until all drainage stops. DRAIN COCK DESCRIPTION Drain   cocks   have   a   brass   body   fitted   with   a tapered  brass  key.    The  drain  cock  is  open  when  the handle   is   parallel   to   the   body   and   closed   when   the handle  is  at  right  angles  to  the  body.    Drain  cocks  are installed in the bottom of each reservoir (Fig.  1) in the air brake system to provide a convenient means of draining the condensation which normally collects in the reservoirs. Always open a drain cock by hand.  Never strike the  handle  with  a  hammer  or  any  other  instrument,  as the cock will be damaged and leakage will develop. Fig.  1  Two Compartment Reservoir and Drain Cocks 1 Supply Reservoir 2 Secondary Reservoir 3 Drain Cocks 4 Automatic Drain Valve SERVICE CHECKS 1. With  air  brake  system  charged,  test  with  soap suds  for  leakage  past  the  key.    Also  check  for leakage through the body by coating the outside of the drain cock with soap CTS-4079 - CHAPTER II PRINTED IN UNITED STATES OF AMERICA Page 3