SERVICE MANUAL the spring above piston "A! forces it into contact with inlet exhaust valve "A" causing the exhaust to seal and inlet to open. Fig. 3  Initial Air System Charge Below 55 PSI 1Primary Reservoir 2Secondary Reservoir 3Inlet & Exhaust A 4Piston A 5Piston B 6Check Valve 7Exhaust 8Inlet & Exhaust B 9Spring Brake Control Valve 10Double Check Valve System Fully Charged (Fig. 4) When the air pressure builds up past the approximate  379  kPa  (55  psi)  in  both  the  primary  and secondary  reservoirs,  piston  "A"  has  moved  against  the force  of  the  spring  above  it  permitting  the  inlet  of  valve "A"   to   close   and   open   the   hollow   exhaust   passage through piston "A." When air pressure under piston "B" is about 655 kPa (95  psi),  piston  "B"  raises  slightly  against  the  force  of springs  above  it  allowing  the  inlet  of  valve  "B"  to  close. The   exhaust   through   valve   "B"   remains   closed.   The closing of  inlet  portion  of  valve  "B"  traps  about  655  kPa (95    psi)    in    the    hold-off    cavity    of    the    spring    brake actuators while allowing full air system pressure to build elsewhere. Normal Brake Application with Primary and Secondary Reservoirs Charged (Fig. 5) When the service brakes are applied by ac- tuating   the   dual   brake   valve,   air   from   the   secondary system  is  delivered  from  the  brake  valve  to  the  control port where it is stopped at the closed inlet valve "A." No further movement of internal components of the inversion valve takes place. Air from the primary delivery system of the brake valve actuates the service brakes of the spring brake chambers. Fig. 4  Air System Fully Charged Fig. 5  Normal Broke Application, Primary and Secondary Systems Charged 1Primary Reservoir 2Secondary Reservoir 3Inlet & Exhaust A 4Piston A 5Piston B 6Check Valve 7Exhaust 8Inlet & Exhaust B 9Spring Brake Control Valve 10Double Check Valve Brake Application with Loss of Air in Secondary System (Fig. 6) If air pressure is lost in the secondary reservoir, the primary   reservoir   as   well   as   the   spring   brake   control valve is protected against air loss through action of single check valve (air source to primary reservoir) and double check valve (Fig. 6). A brake application at brake valve in this situation results in little or no air being delivered from the   secondary   system   to   the   control   port   of   inversion valve.    No  movement  of  the  internal  components  of  the valve takes place.  Braking is assured since the primary reservoir is protected and the primary delivery system of the brake valve will apply the service brake portion of the spring brake chambers. CTS-4079 - CHAPTER VII - Page 4