SERVICE MANUAL Fig.    4    illustrates    the    tractor    protection    valve    in normally closed position for operating the tractor "bobtail" (without trailer). Rubber valve (8) is seated against check valve body (6) by piston (10) closing off passage of air through valve from tractor service ports (H and I) to trailer service port (M).  Cavity  "B"  is  vented  to  exhaust  through  the  Trailer Emergency   Control   (not   shown)   located   in   cab   dash panel.  The   trailer   service   port   (M)   and   cavity   "A"   are vented to atmosphere. After  connecting  the  trailer  and  pushing  the  Trailer Emergency    Control    valve    knob    "in,"    air    pressure    is supplied   from   tractor   system   through   cavity   "B"   (from tractor emergency port "J" to trailer emergency port "L"). Air  pressure  above  276-345  kPa  (40  to  50  psi)  against piston (10) causes it to move towards spring (12). Piston bottoms out on exhaust disc (13) which is held securely in  place   by   snap   ring   (16).   Service   air   passage   from tractor to trailer is open. During normal service brake application, the controlled   air   supplied   by   the   foot   valve   enters   the service ports marked foot valve no.  1 (primary) and foot valve  no.  2  (secondary).    The  air  pressure  moves  valve disc  (3)  against  cover  (2)  closing  off  hand  control  port (G).  Air  is  allowed  to  enter  cavity  "A".  The  air  pressure moves valve (8) down to seat on piston (10) blocking off cavity "C". Air then flows around the diaphragm effect of seal   (8)   and   out   the   trailer   service   port   (M)   to   trailer brakes. The air supplied by the hand control valve enters the   service   port   (G)   marked   "hand   control".   The   air pressure moves valve disc (3) (under cover (2)) against the   seat   on   check   valve   body   (6)    blocking    the    air passage  to  the  foot  valve  port  no.  1  (H).    The  air  flows around   valve   disc   (3)   and   enters   cavity   "D".   The   air pressure pushes valve disc (3) (under cover (4)) against cover (4) blocking the air passage to foot valve port no. 2 (I). The air pressure enters cavity "A" and continues the same   function   as   previously   stated   in   service   brake application. When the service brakes are released, the controlled air   supplied   through   and   downstream   of   the   tractor protection  valve  acts  to  unseat  valve  (8)  from  exhaust seat on piston (10). The exhaust air enters cavity "C" and out    exhaust    port    (K).        Air    upstream    from    tractor protection valve is exhausted through foot valve or hand control valve. If service brake system pressure on the tractor drops to  207-276  kPa  (30  to  40  psi),  spring  force  (12)  moves piston  (10)  towards  inlet  seat  on  check  valve  body  (6) and closes the tractor to trailer service port (M). he  automatic  shut-off  and  bleed  back  feature  functions when   the   service   brakes   or   hand   control   valve   are applied. When the Trailer Emergency dash control valve is pulled out, pressure in cavity "B" will be reduced to "O" kPa ("0" psi), trapping applied service brake pressure in trailer  service  port  (M).  At  this  time,  the  trapped  air  will bleed by one-way check disc (21) into cavity "B" which is exhausted    to    atmosphere    through    trailer    emergency valve at instrument panel. MAINTENANCE Once   each   year   or’   every   160,000   km   (100),00t) miles)   remove,   disassemble   and   clean   all   part4.      All rubber parts must be replaced. SERVICE CHECKS When  the  service  checks  of  the  tractor  protection valve  are  made  on  the  vehicle,  both  the  Leakage  and Performance checks will be accomplished simultaneously. To obtain access to the manifolded tractor protection valve   on   some   vehicles,   removal   of   the   floor   panel directly ahead of the driver’s seat will be required. Unless  otherwise  instructed,  all  disconnect  points  in the   following   instructions   will   be   made   at   the   tractor protection valve. Leakage Test 1. Disconnect  hand  control  (G)  and  trailer  service air   (M)   lines,   Trailer   Emergency   Control   valve pulled out (no air to Trailer Emergency). 2. With 689-861 kPa (100-125 psi) in both primary and   secondary   air   systems,   apply   the   brake (foot)   valve.   Total   leakage   measured   at   hand control   port   (G),   trailer   service   port   (M)   and exhaust  port  (K)  should  not  exceed  82  cm3  (5 cu. in.) per minute or total of a 25 mm (1") soap bubble in 12 seconds. 3. Connect    hand    control    valve    line    to    tractor protection    valve    and    disconnect    brake    (foot) valve lines (H and I). 4. Apply hand control valve. Total leakage measured  at  foot  valve  ports  (H  and  I),  trailer service   port   (M)   and   exhaust   port   must   not exceed 82 cm3 (5 cu. in.) per minute or a total of a 25 mm (1") soap bubble in 12 seconds. 5. Disconnect  lines  at  hand  control  port  (G)  and trailer emergency port (L). 6. Install a pipe plug in trailer emergency port (L). 7. With 689-861 kPa (100-125 psi) in both primary and secondary air systems, push trailer emergency  control  "in".  Leakage  through  valve and  at  trailer  service  (M)  and  exhaust  (K)  ports should not exceed 82 cm3 (5 cu. CTS-4079 - CHAPTER XI - Page 7