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TRUCK SERVICE MANUAL TM 5-4210-230-14&P-1 Fig.  2  External View DESCRIPTION These   self-centering   type   brakes   have   two   single   end wheel    cylinders,    each    operating    a    forward    acting    shoe providing directional braking response.  The closed end of the wheel  cylinder  has  a  slotted  ramp  which  is  the  self-centering sliding anchor for the opposite shoe.  The two wheel cylinder pistons  apply  equal  amounts  of  hydraulic  force  to  each  shoe toe. Shoes are adjustable manually by means of two hex head friction   spring   locked   studs,   exposed   on   the   backing   plate (Fig.  2).  Each stud rotates an adjuster cam located under a brake   shoe   table.      Shoe   hold-downs   consist   of   two   plain washers, multi-rattle wave washer between and a C-clip fitted over  the  extended  shank  of  the  cam  stud.    Two  retracting springs are hooked between the shoes and have their longer shanks connected to the anchor end of the shoes. Upon    brake    application,    the    wheel    cylinder    pistons transmit  pressure  to  the  toes  of  the  shoes,  forcing  the  shoe linings  into  contact  with  the  brake  drum.    If  the  vehicle  is moving forward, the drag of the drum against the shoe lining produces   "self-energization"   which   tends   to   help   rotate   the shoes   outwardly   about   their   anchor   points.      This   action multiplies  the  forces  exerted  against  the  drum  and  produces additional   braking   effect.      Both   shoes   are   forward   acting (primary   shoes),   self-energizing   in   the   forward   direction   of drum rotation. If  the  vehicle  is  moving  backward,  the  drag  of  the  drum on  the  linings  is  in  the  opposite  direction  and  produces  "de- energization" which tends to move the shoe heels away from their anchor blocks.  Piston forces at the shoe toes are large Fig.  3  Internal View enough to overcome this action, but the shoes tend to rotate inwardly  about  their  anchor  points  and  attempt  to  leave  the drum.  Both  shoes  are  reverse  acting  since  neither  is  self- energized in the reverse direction of drum rotation. Cylinder   anchor   block   sides   are   aligned   on   the   axle radius.  As the shoes roll upon their anchor blocks to contact the  drum,  the  heels  may  also  slide  radially  upon  the  anchor block  surface.    The  shoes  thus  automatically  "selfcenter"  in relation to the drum. The   self-energization   factor   causes   this   brake   to   be approximately     three     times     as     effective     during     forward operation as it is during reverse operation; therefore its use is generally confined to the front axle of vehicles in conjunction with  a  rear  axle  brake  of  a  type  providing  effective  stopping ability in reverse as well as forward motion. LINING CLEARANCE ADJUSTMENT Lining  to  drum  clearance  adjustment  is  required  when shoes are relined and, on occasion, to compensate for normal lining  wear.    Clearance  should  be  sufficient  to  avoid  "brake drag" and yet close to afford a good "pedal reserve". Manually operated and vacuum-hydraulic actuated brakes  require  adjustment  (or  relining)  when  pedal  reserve approximates 2"; that is, when the brake pedal drops to within 2" of floor board on hard application. Adjustment is made with the vehicle resting CTS-2078T - Page 3 PRINTED IN UNITED STATES OF AMERICA

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