TM10-3930-660-34 Pressure to operate the brakes is supplied by accumulator (3) Pump (1) flow is used in the brake circuit solely to maintain the accumulator charge.  Pressure switch (4) is used to monitor brake circuit operating pressure and turns on the LOW BRAKE PRESS warning light in the cab if pressure drops to below approximately 650 psi. b. Accumulator  Charging  Cycle  Force  to  operate  the  service  brakes  is  produced  by    and  the  frame  tilt  valve  (not shown)  back  to  the  reservoir  the  precharge  pressure  in  the  accumulator  (3)  When  the  brake  foot  pedal  is depressed,  a  path  is  completed  from  accumulator  (3).    through  ball  valve  (5)  to    wheel  brakes  (6).    Pressure exerted  by  the  accumulator  is  applied  equally  to  the    cylinders  of  all  four  wheel  brakes  (6).    The  path  between accumulator  (5)  and    pump  (1)  is  blocked  at  this  time  by  check  valve  (7),  which  is  seated  by    accumulator pressure.    Check  valve  (7)  will  remain  seated  as  long  as  accumulator    pressure  is  higher  than  pump  pressure Charging valve (8) will then be in the  position shown, and all flow from pump (1) will pass through charging valve (8) and the frame tilt valve (not shown) back to the reservoir. When, during brake operation, accumulator pressure drops below a preset pressure established by design of pilot valve (9), charging valve (8) must be  shifted to permit recharging of the accumulator.  When accumulator pressure reaches  its  lower  limit,  pilot  spring  (10)  will  force  the  spool  in  pilot  valve    (9)  upward  (as  shown)  Accumulator pressure is then applied through ports in  pilot valve (9) to the left end of the spool in charging valve (8), shifting  the spool to the right (as shown).  Since pump pressure is higher than  accumulator pressure at this time, check valve (7) will be unseated and pump  flow is permitted through the charging valve ports and check valve (7) to the  bottom of accumulator (3).  Orifice (11) in the check valve seat restricts the  rate of flow to accumulator (3) Orifice (12) in charging valve (8) limits flow  to frame tilt circuit while charging valve (8) is shifted to allow accumulator recharging. When accumulator pressure again reaches its upper limit, accumulator pressure  combined with force of spring (13) will overcome the force of pilot spring (10)  and shift pilot valve (9) spool downward (as shown).  This will remove  accumulator pressure from the left end of charging valve (8) spool and permit  pump pressure to again shift charging valve spool to the left (as shown).  Check  valve (7) will again be seated by higher accumulator pressure and block the  path between pump (1) and accumulator (3) Section II BRAKE SYSTEM MAINTENANCE PROCEDURES BRAKE MAINTENANCE TASK SUMMARY TASK PAGE PARA PROCEDURES NO 10-3 Disc Brake Assembly - Replace/Repair 10-3 10-4 Brake Control Valve - Repair 10-9 10-2