The power behind your brake pedal is the Toyota brake master cylinder. By being designed to change physical motion to hydraulic pressure, this braking component makes it possible to send out pressurized brake fluid to each of the four brakes on your Toyota when you need to stop your car or truck. The actual action from the brake pedal moves a lever that pushes a rod in the vacuum booster drum which then pushes the piston in the brake master cylinder of your Toyota. As the piston in the master cylinder moves, it compresses the hydraulic fluid in the system that moves the pistons in both the brake calipers and wheel cylinders. The piston in the Toyota master cylinder has a set of polymer seals that makes it possible for the hydraulic fluid to build up pressure and does not allow for the hydraulic fluid to seep back into the reservoirs. As the pressure builds in the brake lines and hoses from the action of the brake master cylinder, the brake shoes and pads are forced against the smooth surfaces of the brake system and create friction which stops your vehicle. The only possible failure of the brake master cylinder is when one or both off the polymer seals on the piston rupture and permits brake fluid to seep past preventing this components ability to build and maintain pressure for the braking system on your Toyota. When this situation occurs, the brake master cylinder has to be replaced or total brake failure will result. The Toyota brake master cylinder is mounted with two studs and nuts on the brake vacuum booster that have to be removed along with the brake lines for this component to be exchanged. When the new part is mounted, then the bleeding of your vehicle is done then it is again safe to take out on the road.