Compensator sleeve Piston rod O-ring and washer Compensator port B A Poppet Piston Toggle Piston rod to brake pedal Spool valve Compensator port Brakes on To system return manifold To system  pressure  manifold To brake wheel cylinder Figure 13-96.  A master cylinder for a boosted brake system  augments foot pedal pressure with aircraft system hydraulic  pressure during heavy braking. Figure 13-95.  A common master cylinder with built-in reservoir is  shown. Illustration A depicts the master cylinder when the brakes  are off. The compensating port is open to allow fluid to expand  into the reservoir should temperature increase. In B, the brakes  are applied. The piston rod-end seal covers the compensating port  as it contacts the piston head.  When the pedal is released, the piston rod travels in the  opposite direction, and the piston returns to the piston stop.  The compensating poppet reopens. The toggle is withdrawn  from the spool via linkages, and fluid pushes the spool back to expose the system return manifold port. System hydraulic  fluid used to boost brake pressure returns through the port.  When the brakes are applied, the pressure from the pilot’s foot  through the mechanical linkage moves the master cylinder  piston in the direction to force fluid to the brakes. The initial  movement closes the compensator poppet used to provide  thermal expansion relief when the brakes are not applied.  As the pilot pushes harder on the pedal, a spring-loaded  toggle moves a spool valve in the cylinder. Aircraft hydraulic  system pressure flows through the valve to the back side of  the piston. Pressure is increased, as is the force developed  to apply the brakes.  Large and high-performance aircraft are equipped with  power brakes to slow, stop, and hold the aircraft. Power  brake actuating systems use the aircraft hydraulic system as  the source of power to apply the brakes. The pilot presses  on the top of the rudder pedal for braking as with the other  actuating systems. The volume and pressure of hydraulic fluid  required cannot be produced by a master cylinder. Instead, a  power brake control valve or brake metering valve receives  the brake pedal input either directly or through linkages.  The valve meters hydraulic fluid to the corresponding brake  assembly in direct relation to the pressure applied to the pedal. 13-55 Power Brakes 