SERVICE MANUAL MIDLAND-ROSS TRACTOR PROTECTION VALVE DESCRIPTION The   tractor   protection   valve   (Fig.   1)   is   normally mounted  at  the  rear  of  cab  where  tractor  air  system  is connected to the service and emergency air hoses used for coupling tractor and trailer brake systems. Valve has a piston arrangement that is pressure sensitive to tractor air  brake  system,  which  provides  automatic  closing  of both lines when system pressure approaches low energy levels. Manual actuation of the valve is performed by the operator   through   use   of   a   two-way   air   control   valve. When  the  control  valve  is  opened  (knob  pushed  in),  air pressure  is  supplied  to  control  port  to  open  the  tractor protection valve. Pulling air control "out" exhausts the air, allowing the tractor protection valve to close. Fig. 1 Tractor Protection Valve 1 Exhaust Port 4 Tractor Service 2 Control Port 5 Trailer Service 3 Tractor Emergency 6 Trailer Emergency A  quick  release  feature  is  also  incorporated  in  the valve    and    serves    to    improve    normal    service    brake release   time   of   trailer   brakes.   During   brake   release, control air delivered to actuate trailer brakes will exhaust at  tractor  protection  valve  as  compared  to  exhausting through foot or hand control valves. If either an automatic or manual operation closes the tractor protection valve, air in the trailer emergency line is exhausted through the tractor protection valve. During normal tractor operation without trailer, the air control   valve   is   "out"   and   tractor   protection   valve   is closed.   Air   passages   leading   from   tractor   system   to trailer couplings are blocked. After coupling a trailer to the tractor, the two-way air control is pushed "in." Air flows through tractor protection valve   emergency   line   to   charge   trailer   brake   system reservoirs.  Trailer brakes are released simultaneously if trailer brakes are holding prior to coupling. During   normal   tractor-trailer   operation,   the   tractor protection  valve  remains  open.  It  allows  air  to  flow  in either   direction   between   trailer   air   brake   system   and tractor air brake system. Trailer    service    brakes    are    applied    and    released    by driver’s operation of either the foot brake control or hand control valve.  Service brake control air flows through the service  passage  of  the  tractor  protection  valve.    Trailer air  reservoirs  and  brake  system  are  replenished  by  air flow through the emergency passage. Driver     can     make     an     emergency     trailer     brake application  by  moving  air  control  valve  "out."  Tractor-to- trailer    service    and    emergency    passages    close    and exhaust  passages  open.    Trailer  brakes  apply  when  air exhausts from the trailer  emergency  line  through  tractor protection valve exhaust passage. The  air  control  valve  is  moved  "out"  to  close  tractor protection valve prior to disconnecting trailer from tractor. OPERATION Fig.  2  illustrates  the  tractor  protection  valve  in  the closed  position  for  operating  a  tractor  without  a  trailer (bobtailed).  Refer  to  Fig.  2  for  numbers  and  letters  in parenthesis. Closed Position (Air Control Valve "Out") Rubber valve (13) is seated against spool (7) closing off passage of air through valve from  tractor  service  (F) to trailer service port (E).  Exhaust valve assembly (5) is seated against valve body (20) closing off passage of air through   valve   from   tractor   emergency   (G)    to    trailer emergency port (D). The control port kH) and  cavity  "A" are  vented  to  exhaust  through  the  two-way  air  control valve  on  instrument  panel.  Both  trailer  emergency  and service ports (D and E) are open to atmosphere through exhaust passages "B  and “C” Open Position (Air Control Valve "In") When   trailer   hoses   are   connected   to   trailer,   air pressure  is  supplied  from  tractor  air  system  to  control port (H) and enters cavity WA.  Air pressure above 345 kPa (50 psi) acts against piston (17) to move it towards spring (19). Piston seats against valve face of assembly (5). Further movement pushes exhaust valve assem- CTS-4079 - CHAPTER XI - Page 3