It is the force exerted by the Toyota drum brake wheel cylinders that push the brake shoes against the drum walls creating the friction necessary to stop your vehicle. Inside of each wheel cylinder are two pistons and two polymer seals. The pistons in the drum brake wheel cylinders are pushed out by the hydraulic force that is provided through the brake lines. These lines are attached to the brake master cylinder under the hood of your Toyota. If you are looking at the drum brake with the drum removed, the wheel cylinder is located in the center upper most section of the backing plate. The only problem a Toyota owner will have with this braking component is when the polymer seal that holds the hydraulic fluid in the cylinder wears out or cracks over time. This permits the hydraulic fluid to leak out of its closed loop system and reduces the effectiveness of the braking procedure. The only solution when there is a leak from the Toyota drum brake wheel cylinder is having it rebuilt or replaced. The replacement procedure is the same as a regular brake job except there are three additional steps. Once the drum brake shoes are removed, the rear brake line leading into the wheel cylinder should be removed. Then you can unbolt the cylinder itself. By removing the line first, you will prevent any damage from occurring to it. It can be very stubborn to remove since it has been in place for some time. With the new braking component in place the braking system on your Toyota should then be bled of any trapped air that might have entered it and the vehicle will be ready for the road once more.