Pressure comparator Pressure comparator Left gear Right gear M 1 2 3 1 3 2 1 2 3 1 2 3 M “A” system pressure Charging valve System “A” solenoid valve System “B” solenoid valve To system return Accumulator Transmitter Sealing disconnect fitting (type) System return System pressure Brake pressure Outboard wheel Inboard wheel Swivel (type) Gauge Main landing gear “UP” line Automatic wheel snubbing Brake valve  feel augmentation  actuator Brakes ON Parking brake  shutoff valve “B” system pressure Ground interconnect valve Brake metering valve Antiskid valve Shuttle valve Figure 13-99.  The power brake system on a Boeing 737.  Pressure = Force/Area High-pressure hydraulic system input pressure acts on the  small end of a piston. This develops a force proportional to  the area of the piston head. The other end of the piston is  larger and housed in a separate cylinder. The force from the  smaller piston head is transferred to the larger area of the  other end of the piston. The amount of pressure conveyed  by the larger end of the piston is reduced due to the greater  area over which the force is spread. The volume of output  fluid increases since a larger piston and cylinder are used.  The reduced pressure is delivered to the brake assembly.  The spring in the debooster aids in returning the piston to  the ready position. If fluid is lost downstream of the deboost  cylinder, the piston travels further down into the cylinder  when the brakes are applied. The pin unseats the ball and  allows fluid into the lower cylinder to replace what was lost.  Once replenished, the piston rises up in the cylinder due  to pressure build-up. The ball reseats as the piston travels  above the pin and normal braking resumes. This function is  not meant to permit leaks in the brake assemblies. Any leak  discovered must be repaired by the technician.  A lockout debooster functions as a debooster and a hydraulic  fuse. If fluid is not encountered as the piston moves down in  the cylinder, the flow of fluid to the brakes is stopped. This  prevents the loss of all system hydraulic fluid should a rupture  downstream of the debooster occur. Lockout deboosters have  a handle to reset the device after it closes as a fuse. If not  reset, no braking action is possible.  13-59