The master cylinder is connected to the brake pedal via a pushrod.
This is a single master cylinder for a drum brake system. Its one piston has a primary and a secondary cup. These are also known as seals, because, when force is applied to the brake pedal, the primary cup seals the pressure in the cylinder. The secondary cup prevents loss of fluid past the end of the piston. An outlet port links the cylinder to the brake lines. An inlet port connects the reservoir with the space around the piston. A compensating port connects the reservoir to the cylinder, ahead of the primary cup.
With the brakes off, this port connects the brake system with the reservoir. It compensate for changes in volume of the fluid due to heat or wear.
The rod from the brake pedal pushes on the piston. It moves, closing off the compensating port and trapping fluid ahead of the primary cup. Any fluid trapped in the cylinder is then forced through a valve, called a residual pressure valve, into the brake lines. When the brakes are released, the master cylinder piston returns to its original position.
When the piston fully returns against its stop, the primary cup uncovers the compensating port. Fluid ahead of the primary cup can now return this way to the reservoir.
When the pedal is released quickly, the spring makes the piston return quickly. But the fluid cannot return as quickly to the cylinder. A low-pressure area develops ahead of the primary cup, which could draw air into the system. To prevent this, small holes are drilled in the piston. Fluid from the reservoir can pass through the inlet port and past the edge of the primary cup. This is called recuperation.
When the fluid in the lines returns to rest, its pressure is held above atmospheric pressure, by a valve called the residual line pressure valve. The residual pressure helps stop air from entering, at the wheel cylinder. And it keeps fluid from leaking out.
