Valve body Springseat Compensating chamber Return spring Brake metering valve slide Brake metering valve sleeve Autobrake piston Link assembly Brake input shaft Crank Crank stop To  outboard  brake To  return Autobrake To  inboard brake To  return “A” system pressure “B” system pressure Figure 13-98.  A brake metering valve from a Boeing 737. A machined slide or spool moves laterally to admit the correct amount of  hydraulic system fluid to the brakes. The pressure developed is in proportion to the amount the rudder/brake pedal is depressed and the  amount the slide is displaced. The slide/spool also simultaneously controls the return of fluid to the hydraulic system return manifold  when brake pressure is released.  side of the diaphragm to operate the brakes in case of an  emergency. It is forced out of the accumulator into the brakes  through the system lines under enough stored pressure to slow  the aircraft. Typically, the accumulator is located upstream of  the brake control/metering valve to capitalize on the control  given by the valve.  [Figure 13-101] At the same time, a shut-off valve is closed in the common  return line from the brakes to the hydraulic system. This  traps the fluid in the brakes holding the rotors stationary.  Depressing the pedals further releases the pedal ratchet and  opens the return line valve. Some aircraft brake assemblies that operate on aircraft  hydraulic system pressure are not designed for such high  pressure. They provide effective braking through a power  brake system but require less than maximum hydraulic  system pressure. To supply the lower pressure, a brake  debooster cylinder is installed downstream of the control  valve and anti-skid valve.  [Figure 13-103]  The debooster  reduces all pressure from the control valve to within the  working range of the brake assembly. Some simpler power brake systems may use an emergency  source of brake power that is delivered directly to the brake  assemblies and bypasses the remainder of the brake system  completely. A shuttle valve immediately upstream of the  brake units shifts to accept this source when pressure is lost  from the primary supply sources. Compressed air or nitrogen  is sometimes used. A pre-charged fluid source can also be  used as an alternate hydraulic source.  The parking brake system function is a combined operation.  The brakes are applied with the rudder pedals and a  ratcheting system holds them in place when the parking  brake lever on the flight deck is pulled.  [Figure 13-102] Brake deboosters are simple devices that use the application  of force over different sized pistons to reduce pressure.  [Figure 13-104]  Their operation can be understood through  the application of the following equation:  Brake Deboosters Parking Brake 13-58