SERVICE MANUAL 12. Install lower static piston assembly in valve body. Then  install  retainer  (7)  making  certain  locking tabs engage on valve body bosses. 13. Install  pedal   assembly   using   three   capscrews. Check  to  be  certain  plunger  is  in  contact  with spring seat.  The stop button should be adjusted so    that    the    roller    and    plunger    contact    after adjustment;  roller  should  be  able  to  be  turned freely by thumb. 14. Test rebuilt brake valve by performing operation and    leakage    tests    as    outlined    in    SERVICE CHECKS. MIDLAND-ROSS DESCRIPTION The    Midland-Ross    dual    air    control    valve    is    a    foot operated   type   brake   valve.      The   valve   may   be   floor mounted  with  a  treadle  assembly  or  it  may  be  firewall mounted with a bracket assembly for a suspended pedal. The    valve    has    two    separate    reservoir    and    delivery circuits    which    consist    of    two    separate    piston    type compensating  valves.    The  primary  section  of  the  valve responds to mechanical force from the operator and the function is similar to a single-circuit  foot  operated  valve. The  secondary  section  of  the  valve  resembles  a  relay’ valve  and  responds  to  air  pressure  delivered  from  the primary    section.        During    normal    operation,    similar pressure levels are delivered to both circuits of the  dual air  brake  system  which  are  supplied  from  separate  air pressure  reservoirs.      When   the   valve   is   released,   air from   both   circuits   is   exhausted   at   the   exhaust   port. During a condition of insufficient air supply in the primary section the secondary section function is similar to a foot operated valve. Both primary and secondary sections  of the brake valve use a common exhaust valve. OPERATION Refer   to   Fig.   10   for   numbers   in   parenthesis.   Fig.   10 illustrates    the    normal    position    of    the    brake    valve components  with  the  vehicle  service  brakes  in  the  fully released    position.        Basically,    the    same    operational illustrations  (Figs.    3,  4,  5,  6  and  7)  which  apply  to  the Bendix brake valve will apply to the Midland-Ross brake valve.          Application     of     Primary     Section     -     Normal Operation When    the    foot    valve    actuator    (treadle    or    pedal)    is depressed,  the  plunger  exerts  force  against  the  spring retainer  (1),  compensatory  spring  (32)  and  the  primary piston (31).  The primary pis ton  moves  and  its  exhaust  seat  "B"  seals  against  the rubber   face   of   seal   assembly   (29).      Further   piston movement causes the retainer and seal assembly (8 and 29)  to  move  away  from  the  inlet  seat  of  valve  body  (4) allowing  primary  reservoir  pressure  "D"  to  enter  cavity "A"   and   flow   out   the   primary   delivery   port.      As   air pressure  builds  up  in  cavity  "A,"  it  acts  against  piston (31)  causing  it  to  move  upward.    When  the  resultant forces  on  the  piston  approach  a  balanced  position,  the piston moves upward until the  seal  assembly  (29)  seals against the inlet seat of valve body (4).  The exhaust seat "B" of piston (31) remains closed and air is blocked from exhausting into cavity "C." The valve has then reached a balanced or "lap" condition. Application of Secondary Section - Normal Operation When the primary delivery port is pressurized, air passes through the bleed hole "E" into the control portion of the secondary   section   "F."   The   air   pressure   causes   the secondary piston (23) to  move  and  its  exhaust  seat  "G" seals   against   the   rubber   face   of   seal   assembly   (16). Further  piston  movement  causes  the  retainer  and  seal assembly (8 and 16) to move away from the inlet seat of the   valve   body   (21)   allowing   the   secondary   reservoir pressure    "J"    to    enter    cavity    "H"    and    flow    out    the secondary   delivery   port.   As   air   pressure   builds   up   in cavity "H," it acts against secondary piston (23) causing it to   move   upward.      When   the   resultant   forces   on   the piston approach a balanced condition, the piston moves upward  until  the  seal  assembly  (16)  seals  against  the body (20).  The exhaust seat "G" of piston (23)  remains  closed  and  air  is  blocked  from  exhausting into cavity "K." The valve has then reached a balanced or "lap"   condition.      The   delivery   pressure   of   secondary section    is    then    approximately    equal    to    the    delivery pressure of primary section. Application - Loss of Secondary Section Air Pressure When air pressure is lost in the secondary  section.    the primary  section  functions  as  described  in   the   "normal operation" of the primary section. Application - Loss of Primary Section Air Pressure When    air    pressure    is    lost    in    the    primary    section, depressing  the  valve  actuator  (treadle  or  pedal)  causes the  plunger  to  exert  force  against  n  the  spring  retainer (1),   compensatory   spring   (32)   and   the   primary   piston (31).  The primary piston exerts force against  the  upper stem  of  the  secondary  piston  (23)  and  its  exhaust  seat "G"   seals   against   rubber   face   of   seal   assembly   (16). Further piston movement causes the retainer and seal CTS-4079 CHAPTER IV - Page 11