Many power brake system designs are in use. Most are  similar to the simplified system illustrated in  Figure 13-97A .  Power brake systems are constructed to facilitate graduated  brake pressure control, brake pedal feel, and the necessary  redundancy required in case of hydraulic system failure.  Large aircraft brake systems integrate anti-skid detection  and correction devices. These are necessary because wheel  skid is difficult to detect on the flight deck without sensors.  However, a skid can be quickly controlled automatically  through pressure control of the hydraulic fluid to the brakes.  Hydraulic fuses are also commonly found in power brake  systems. The hostile environment around the landing gear  increases the potential for a line to break or sever, a fitting to  fail, or other hydraulic system malfunctions to occur where  hydraulic fluid is lost en route to the brake assemblies. A  fuse stops any excessive flow of fluid when detected by  closing to retain the remaining fluid in the hydraulic system.  Shuttle valves are used to direct flow from optional sources  of fluid, such as in redundant systems or during the use of  an emergency brake power source. An airliner power brake  system is illustrated in  Figure 13-97B .  the slide. This acts on the end of the slide creating a return  force that counters the initial slide movement and gives feel  to the brake pedal. As a result, the pressure and return ports  are closed and pressure proportional to the foot pressure on  the pedal is held on the brakes. When the pedal is released,  a return spring and compensating chamber pressure drive  the slide to the right into its original position (return port  open, supply pressure chamber and brake pressure chambers  blocked from each other).  The metering valve operates as described simultaneously  for the inboard and the outboard brakes.  [Figure 13-98]  The  design of the link assembly is such that a single side of the  metering valve can operate even if the other fails. Most brake  control valves and metering valves function in a similar  manner, although many are single units that supply only one  brake assembly.  The auto brake, referenced in the metering valve diagram,  is connected into the landing gear retraction hydraulic line.  Pressurized fluid enters this port and drives the slide slightly  to the left to apply the brakes automatically after takeoff. This  stops the wheels from rotating when retracted into the wheel  wells. Auto brake pressure is withheld from this port when  the landing gear is fully stowed since the retraction system  is depressurized. The majority of the rudder/brake pedal feel is supplied by  the brake control or brake metering valve in a power brake  system. Many aircraft refine the feel of the pedal with an  additional feel unit. The brake valve feel augmentation  unit, in the above system, uses a series of internal springs  and pistons of various sizes to create a force on the brake  input shaft movement. This provides feel back through the  mechanical linkages consistent with the amount of rudder/ brake pedal applied. The request for light braking with slight  pedal depression results in a light feel to the pedal and a  harder resistance feel when the pedals are pushed harder  during heavy braking.  [Figure 13-100]  A brake metering valve from a Boeing 737 is illustrated in  Figure 13-98 . The system in which it is installed is diagramed  in  Figure 13-99 . Two sources of hydraulic pressure provide  redundancy in this brake system. A brake input shaft,  connected to the rudder/brake pedal through mechanical  linkages, provides the position input to the metering valve.  As in most brake control valves, the brake input shaft moves  a tapered spool or slide in the valve so that it allows hydraulic  system pressure to flow to the brakes. At the same time, the  slide covers and uncovers access to the hydraulic system  return port as required.  The key element in a power brake system is the brake control  valve, sometimes called a brake metering valve. It responds to  brake pedal input by directing aircraft system hydraulic fluid  to the brakes. As pressure is increased on the brake pedal,  more fluid is directed to the brake causing a higher pressure  and greater braking action.  When the rudder/brake pedal is depressed, the slide in the  metering valve moves to the left.  [Figure 13-98]  It covers  the return port so pressure can build in the brake system. The  hydraulic supply pressure chamber is connected to the brake  system pressure chamber by the movement of the slide, which  due to its taper, unblocks the passage between these two. As  the pedal is depressed further, the valve slide moves farther  to the left. This enables more fluid to flow to the brakes due  to the narrowing shape of the slide. Brake pressure increases  with the additional fluid. A passage in the slide directs brake  pressure fluid into a compensating chamber at the end of  As can be seen in  Figure 13-99,  the brake metering valves not  only receive hydraulic pressure from two separate hydraulic  systems, they also feed two separate brake assemblies.  Each main wheel assembly has two wheels. The inboard  wheel brake and the outboard wheel brake, located in their  respective wheel rims, are independent from each other.  In case of hydraulic system failure or brake failure, each  is independently supplied to adequately slow and stop the  aircraft without the other. More complicated aircraft may  involve another hydraulic system for back-up or use a similar  alternation of sources and brake assemblies to maintain  braking in case of hydraulic system or brake failure.  13-56 Emergency Brake Systems  Brake Control Valve/Brake Metering Valve