SERVICE MANUAL SPRING BRAKE CONTROL VALVE (MIDLAND ROSS) DESCRIPTION The    spring    brake    control    valve    is    a    manually operated   valve   of   push-pull   type.   When   the   valve   is pushed  "in"  the  valve  is  open  to  supply  air.  The  "out" position  closes  off  supply  air  and  allows  delivered  air  to exhaust   to   atmosphere,   applying    the    spring    brakes. Valve also features a plunger pressure sensing arrangement   which   provides   automatic   return   to   "out" position when brake system air pressure approaches low energy levels. Fig. 1 Spring Brake Control Valve OPERATION The  spring  brake  control  valve  serves  to  apply  and release the spring actuated parking brakes.  The valve is in  the  "in"  position  under  normal  operation  on  either  a straight truck  or  tractortrailer.  Air  supply  passes  through the valve delivering  system  pressure  to  retract  and  hold parking  brakes  in  released  position.  Manual  pull  "out" closes     off     air     supply     and     vents     delivered     air     to atmosphere, applying the parking brakes. When valve is pushed "in," it will remain in as long as supply  line   is   48-241   kPa   (7-35   psi)   or   more.   During normal  operation  the  air  control  valve  will  automatically apply, moving "out," if air pressure should drop to 48-241 kPa (7-35 psi). The automatic application will result when both  primary  and  secondary  systems  have  depleted  air supply to 48-241 kPa (7-35 psi). Refer  to  Fig.  2  for  following  operational  description. Fig. 2 illustrates valve in applied (out) position. Pushing  piston  in  moves  valve  assembly  off  body seat  until  contacting  end  cap  seat.  Air  pressure  at  inlet has    free    passage    to    outlet    and    is    blocked    from exhausting to atmosphere. Pulling piston out moves valve  assembly  away  from end  cap  seat  and  contacts  body  seat.  Air  pressure  at inlet  is  blocked  from  entering  either  outlet  or  exhaust passages.    Air  pressure  in  outlet  has  free  passage  to exhaust to atmosphere. The  spring  in  the  valve  assists  to  move  piston  out automatically    if    inlet    pressure    drops    and     effective opposing force across the valve seat is overcome. Fig. 2 Cross Section of Spring Brake Control Valve 1 End Cap 9 Valve Assembly 2 Piston 10    Outlet 3 Inlet 11    "O" Ring 4 Body 12    Snap Ring 5 Name Plate 13    Spring 6 Knob 14    Exhaust 7 "O" Ring 15    Washer 8 Torque Nut to 12 to 17 Inch lbs. MAINTENANCE Once   each   year   or   every   160,000   km   (100,000 miles) the valve should be removed, disassembled and a repair kit installed. SERVICE CHECKS Leakage Test Use    air    pressure    source    equipped    with    in-line manual  shut-off  valve,  air  gauge  known  to  be  accurate and connection.  Connect air source to CTS-4079 - CHAPTER VII - Page 3