SERVICE MANUAL MIDLAND-ROSS RELAY VALVE DESCRIPTION The  Midland  Ross  air  relay  valve  is  a  piston  type,  air operated  valve  used   to   deliver   and   exhaust   air   being supplied   to   brake   chambers.      It   is   also   designed   to provide  a  quick  means  for  applying  and  releasing  the service  brakes.    The  valve  is  remotely  controlled  by  air pressure from the foot valve. When the brake valve is applied the relay valve permits direct  air  pressure,  supplied  from  the  air  reservoir,  to pass and apply the air chambers.  When the brakes are released  the  relay  valve  also  permits  the  air  chamber pressure to exhaust out the valve into the atmosphere. The  air  pressure  from  brake  valve  is  also  influenced  by the action of modulator valve in the anti-lock control. Fig. 1 Midland-Ross Relay Valve 1 Control Port 2 Delivery Ports 3 Reservoir Port OPERATION The  relay  valve  is  mounted  on  the  frame  close  to  the brake  chambers  it  serves.    Fig.  2  illustrates  the  relay valve when the service brakes are fully released. When the brake valve is applied air pressure enters the relay  valve  at  the  control  port  and  into  cavity  "A."    Air pressure   above   piston   moves   it   down   and   seats   the exhaust    valve    against    modulation    tube    closing    off exhaust    passages.    Further    downward    movement    of piston   forces   modulation   tube   down   and   opens   inlet valve    by    moving    away    from    valve    seat    of    body. Reservoir air pressure enters cavity "B" through reservoir port and then passes through delivery ports (not shown) and to service brake chambers. Fig. 2 Cross Section of Midland-Ross Relay Valve 1 Control Port 2 Exhaust Valve Assy. 3 Piston 4 Modulation Tube 5 Valve Body 6 Inlet Valve Assy. 7 Reservoir Port 8 Exhaust Port 9 Washer 10 Retaining Ring 11 Exhaust Valve Body 12 Exhaust Valve Spring 13 Reservoir Port 14 Cavity "B" 15 Cavity "A" Air  is  blocked  from  exhausting  through  modulation  tube center.    As  air  pressure  builds  in  cavity  "B,"  it  tends  to move piston upward.  When the air pressure above and below  piston  approaches  a   balanced   condition,   piston moves upward until inlet valve closes against valve body seat. If   air   pressure   coming   into   the   control   port   is   held momentarily  at  braking  condition,  the  relay  valve  attains a "lap" position.  Lap position implies that both inlet and exhaust  valves  are  seated  simultaneously.    When  the valve  is  in  this  "lap"  position  it  is  instantly  responsive  to any  change  in  pressure  from  the  brake  control  valve  to increase   or   decrease   vehicle   braking.      Air   pressure outputs   of   relay   valve   are   directly   proportional   to   the signal air pressure at control port. CTS-4079 - CHAPTER VI -PAGE 2