Braking Ratio Braking   ratio   refers   to   the   comparison   of front-wheel  to  rear-wheel  braking  effort.  When  a vehicle  stops,  its  weight  tends  to  transfer  to  the  front wheels.  The  front  tires  are  pressed  against  the  road with greater force. The rear tires lose some of their grip on the road. As a result, the front wheels do more of the braking than the rear. For this reason, many vehicles have disc brakes on the front and drum brakes on the rear. Disc brakes are capable of producing more stopping effort than drum brakes. If drum brakes are used on both the front and rear wheels, the front shoe linings and drums typically have a larger surface area. Typically,   front-wheel   brakes   handle   60   to   70 percent of the braking power. Rear wheels handle 30 to 40 percent of the braking. Front-wheel drive vehicles, having  even  more  weight  on  the  front  wheels,  have even a higher braking ratio at the front wheels. HYDRAULIC   SYSTEM The hydraulic system applies the brakes at all four wheels  with  equalized  pressure.  It  is  pedal  operated. The system consists of the master cylinder, the wheel cylinder, the brake lines and hoses, and the brake fluid. Master Cylinder The master cylinder is the primary unit in the brake system  that  converts  the  force  of  the  operator's  foot into fluid pressure to operate the wheel cylinders. It is normally  mounted  to  the  firewall,  which  allows  for easy  inspection  and  service,  and  is  less  prone  to  dirt and water. The master cylinder has four basic functions that are as follows: It develops pressure, causing the wheel cylinder pistons to move towards the drum or rotor. After all of the shoes or pads produce sufficient friction,  the  master  cylinder  assists  in  equalizing the pressure required for braking. It  keeps  the  system  full  of  fluid  as  the  brake linings  wear. It  can  maintain  a  slight  pressure  to  keep contaminants (air and water) from entering the system. In its simplest form, a master cylinder consists of a housing,  a  reservoir,  a  piston,  a  rubber  cup,  a  return spring,  a  rubber  boot,  and  a  residual  pressure  check valve  (fig.  7-4).  There  are  two  ports  (inlet  port  and compensating  port)  drilled  between  the  cylinder  and reservoir.  The  description  of  the  components  of  a master cylinder is as follows: The master cylinder housing is an aluminum or iron  casting  having  either  an  integral  or  detachable reservoir. A cylinder is machined in the housing of the master  cylinder.  The  spring,  the  cups,  and  the  metal piston move within this cylinder. The piston is a long spoonlike member with a rubber secondary cup seal at the outer end and a rubber primary  cup  at  the  inner  end,  which  are  used  to pressurize the brake system. The primary cup is held against the end of the piston by the return spring. A steel stop  disc,  held  in  the  outer  end  of  the  cylinder  by  a retainer spring, acts as a piston stop. Figure 7-4.—Cutaway view of a single master cylinder. 7-3