pressure or return port of the brake line. Two springs are provided. The large spring, referred to in the illustration as the plunger spring, provides “feel” to the brake pedal. The small spring returns the spool to the OFF position. When the plunger is depressed, the large spring moves the spool, which closes off the return port and opens the pressure port to the brake line. When the pressure enters the valve, fluid flows to the opposite end of the spool through a hole. The pressure pushes the spool back far enough toward the large spring to close the pressure port, but not open the return port. The valve is then in the static condition. This movement partially compresses the large spring, giving “feel” to the brake pedal. When the brake pedal is released, the small spring moves the spool back, opening the return port. This action allows fluid pressure in the brake line to flow out through the return port. Maintenance  of  the  sliding  spool  brake  control valve  is  limited  to  checking  the  action  of  the  plunger. This is done by manually depressing the plunger until it bottoms, and then releasing it suddenly. If the plunger remains depressed (does not snap out), the valve is binding at the spool and sleeve. If binding occurs, the valve  should  be  replaced.  Disassembly  of  the  valve  is not permitted at the organizational level of maintenance, but may be performed by an intermediate or higher level activity. Brake  Debooster  Cylinder In  some  power  brake   control   valve   systems, debooster cylinders are used in conjunction with the power  brake  control  valves.  These  units  are  generally used on aircraft equipped with a high-pressure hydraulic system and low-pressure brakes. The purpose of the brake debooster cylinder is to reduce the pressure to the brake and increase the volume of fluid flow. Figure 12-29  shows  a  typical  debooster  cylinder  installation. The unit is being mounted on the landing gear shock strut in the line between the control valve and the brake. The schematic diagram in the illustration shows the internal parts of the cylinder. When the brake is applied, fluid under pressure enters the inlet port to act on the small end of the piston. The ball check prevents the fluid from passing through the shaft. Force is transmitted through the small end of the piston to the large end of the piston. As the piston moves downward in the housing, a new flow of fluid is created from the large end of the housing through the outlet port to the brake. Because the force from the small piston head is distributed over the greater area of the 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. Emergency system pressure  line Main brake pressure line Upper support clamp Packing Packing Debooster cylinder assembly Piston Piston return spring Packing Lower support clamp Riser tube Packing Tee  fitting Brake line (to pressure 15. 16. 17. 18. 19. 20. 21. 22.  Snapring 23. Spring retainer 24. Valve spring 25. Ball 26. Ball pedestal 27.  Barrel 28. Lower cud cap Brake  pressure-relief valve Overflow  line Brake tine (debooster to shuttle valve) Shock strut Torque link Brake shuttle valve Inlet port relief  valve) 29. Outlet port Figure  12-29.-Brake  debooster  cylinder. large piston head, pressure at the outlet poet is reduced. At the same time, a greater volume of fluid is displaced by the large piston head than that used to move the small piston  head. Normally, the brake will be fully applied before the piston has reached the lower end of its travel. However, if the piston fails to meet sufficient resistance to stop it (due to a loss of fluid from the brake unit or connecting lines), the piston will continue to move downward until the riser unseats the ball check valve in the hollow shaft. With the ball check valve unseated, fluid from the power control  valve  will  pass  through  the  piston  shaft  to 12-34