one-way restrictors installed in the open and close lines ensure  smooth  door  operation  and  prevent  cavitation  of the  door-actuating  cylinders. WINDSHIELD  WIPER  SYSTEM The  windshield  wiper  system  shown  in  figure 12-68  consists  of  a  pressure  reducer,  speed  control needle   valve   and   drive   mechanism,   hydraulic actuator,  two  window  actuator  units  and  wiper  blade assemblies,  and  a  return  line  check  valve.  System pressure  is  directed  to  the  pressure  reducer,  where  it is reduced to 2,000 psi, and then the fluid passes to the  speed  control  valve,  which  starts,  stops,  and controls  the  wiper  blade  speed. Hydraulic  fluid  is  directed  from  the  speed  control unit to the hydraulic actuator, which, in turn, controls and directs fluid to the window units. The actuator alternately allows fluid flow to opposite sides of the window  unit  double  piston.  Constant  speed  of  the wiper  blades  is  provided  by  fluid  from  the  speed control  valve  and  is  directed  to  the  balance  pistons  in the hydraulic actuator. Fluid is also directed to the window units through the hydraulic actuator normal inlet port. The window units, by action of a rack and piston arrangement, convert the linear motion of the double piston to the reciprocating action of the drive shaft. When  the  system  has  completed  one  wiper  stroke and the hydraulic pressure at the window unit pistons reaches a value equal to system pressure minus 200 psi,  the  actuator  will  then  reverse  the  flow  to  the opposite side of the window unit piston and repeat the wiper  stroke  action  in  reverse. Any obstruction on one windshield will stop that blade,  but  allow  the  other  to  continue  until  it completes  its  stroke  (or  meets  an  obstruction),  at which  time  the  pressure  in  the  window  units  buildup and the actuator reverses the action of both blades. The  mechanical  locking  device  is  provided  to  hold  the blades in the parked position when the needle valve is closed. NOTE: Do not operate the windshield wiper blades on a dry windshield. Maintenance  of  the  windshield  wiper  system consists mainly of operational checks, removal and installation  of  components,  and  troubleshooting. The  operational  check  should  be  performed according  to  the  following  procedures: 1. Provide a supply of water on the outside surface of the panels when the wiper blades are in motion. 2. Check for a wiper arm force of 7 to 10 pounds on the windshield (at the blade attachment). 3.  Connect  external  electrical  power  supply. 4.  Energize  hydraulic  power  system  No.  1  ac pumps. 5.  Slowly  open  the  windshield  wiper  speed control  needle  valve. 6. Blades must move from parked position and begin to cycle between 100 to 300 strokes per minute. 7.  Open  instrument  panel  to  gain  access  to window units. Bleed air from units as they cycle by cracking the B-nuts on the tubing at each end of the window units. Allow fluid to bleed into existing drip pans until it is evident that all air has been removed. 8. Check that no hydraulic fluid leak is visible on the system tubing, connections, or at any component. 9.  Check  that  system  components  perform smoothly  with  no  erratic  operation  and  blade  reversing is synchronized. Blade rotation must be 75 degrees. The wiper blades must not touch the center post, travel into the  parking  area,  or  short  cycle  during  high-speed operation. 10.  Reduce  speed  of  blade  operation,  and  manually stall each wiper separately. While the blade is stalled, the  opposite  blade  should  operate  smoothly. 11. Park the blades by slowing down the cycling speed  to  permit  blades  to  move  into  the  park  position before  they  reverse. NOTE: Parking  area  is  the  area  between  the bottom edge of the glass and the break in the contour  of  the  fuselage.  To  adjust  the  blades, loosen the blade attaching screw and rotate blade. One serration is approximately 5 degrees of rotation. If it is not possible to install within the parking area, install the arm outboard with the  blades  as  close  to  the  parking  area  as possible, then remove the arm and adapter. Looking down on the arm, carefully remove the  adapter  and  rotate  it  one  serration 12-79