by Richard Rowe
disc brakes image by KtD from Fotolia.com
Hydraulic systems are made up of three basic parts: a master cylinder (piston) that pushes fluid through lines, lines that carry fluid, and a slave cylinder that moves when fluid pressure pushes it. Modern “tandem” master cylinders use a pair of pistons in the same tube that control two different fluid circuits for redundancy that no single piston design can offer.
Individual cylinders are the most basic type of master cylinder and are internally very similar to a plastic medical syringe. The brake pedal lever pushes the plunger (piston) into the cylinder, which pushes the fluid through the lines and into the slave cylinders. When the brake pedal is released, a spring inside the cylinder pushes the plunger back to its original position. The negative pressure pushes the brake fluid into the cylinder from the lines and from the brake fluid reservoir. Automakers switched to the more redundant tandem master cylinder long ago, but many race car builders prefer to use a pair of individual cylinders rather than a single tandem cylinder to control front / rear brake pressure.
Tandem cylinder ported
A tandem cylinder is two pistons in one. The primary piston is connected to the brake pedal. When the brake pedal is depressed, the piston pushes a spring attached to the rear of the secondary piston. Once the spring is fully compressed, the secondary piston begins to push fluid through its own dedicated system. The inlet port of the reservoir allows fluid to flow behind the pistons to maintain uniform pressure on both sides. When the brake pedal is released, spring pressure pushes the pistons back and a small equalization port in the brake fluid reservoir draws additional fluid into the chamber. The equalization port is necessary to accelerate the release of the brake, which would otherwise be inhibited by the velocity of the fluid moving back through the lines.
Master cylinder without port
First introduced in the Toyota MR2, portless master cylinders offer faster brake release than standard designs that use a trim port. Portless cylinders use a valve assembly in the pistons that opens to equalize pressure when the brakes are released. This allows the brake cylinder to bypass the equalization port, which is more restrictive to fluid flow and bleeds pressure from the brake system on initial application. The faster response portless cylinder works best with anti-lock braking systems (ABS), which use fast pressure modulations to adjust braking force.