Everyone who works on brakes MUST know these concepts and procedures In the early days of the automobile, some very clever engineering was employed to apply brakes mechanically. For example, the Italian Bugatti routed the cables over the top of the front axle so that the twisting action generated by stopping added force to the shoe cam lever. No matter how ingenious the design, however, there was always a major drawback: Nothing could insure that braking force would be exactly equal at any pair of wheels, so there was a good chance that stepping on the pedal would cause swerving and skidding.
This made the idea of hydraulically actuated brakes attractive–according to Pascal’s Law, pressure at all points in a closed hydraulic system must necessarily be the same–but it took many years to develop dependable systems. The first car of any consequence to carry four-wheel hydraulic brakes was the. 1921 Dusenberg of the U. S. Basic idea On the most basic level, all brake hydraulic systems share the same principle: Muscle strength amplified by leverage and perhaps a power booster displaces fluid from the master cylinder and causes pressure to increase all through the circuits.
This overcomes the retracting springs in drums and the seals’ elasticity in disc calipers and pushes the friction material against the rotating member. That much is obvious, but the subtleties of modern designs that provide proper performance in the real world deserve some explanation. Hence this article, which also includes important service information every mechanic should know about. Dual master Although it has been in use for decades all over the world, the dual (also called \”split\” or \”tandem\”) master cylinder is still widely misunderstood, so we had better explain its construction and operation.
A typical late-model specimen will be of the composite variety (aluminum with a plastic reservoir), but iron one-piece units are still around in abundance. Two pistons ride in the bore, and here is where we encounter some confusing terminology. The rear piston is the primary, and the one in the front is the secondary. This apparent misnaming resulted because the rear piston is the first to receive the force of the driver’s leg. Each piston has a primary seal at its front and a secondary at its rear, so you will be hearing such combinations as primary piston secondary seal, secondary piston secondary seal, etc.
The primary seals are the most important because they trap the fluid that is about to be squeezed into the lines. The primary piston’s secondary seal keeps fluid from escaping out of the back of the cylinder (commonly into a booster), and the secondary piston’s secondary seal acts as a barrier to make two essentially separate cylinders out of one. In normal braking, the push rod from the pedal or booster forces the primary piston forward. No pressure is created until the primary seal covers the compensating or vent port from the reservoir.
Once it does, fluid is trapped in the chamber between the pistons and becomes, for all intents and purposes, a solid column. Pressure is routed from this chamber to two wheels. The trapped fluid and the primary piston coil spring both bear on the secondary piston, moving it forward and creating pressure in the chamber ahead of the secondary piston’s primary seal, to which the line to the other two wheels is attached. Continued from page 1
When the pedal is released, a partial vacuum occurs in both pressure chambers because the fluid’s inertia and viscosity prevent it from returning from the lines immediately. In order to re-arm the brakes instantaneously, the primary seals are designed to allow fluid to flow one way (forward) from behind each seal into the pressure chambers. The replenishing ports allow fluid to move freely between the chambers behind both pistons’ primary cups and the reservoir according to demand and expansion and contraction from temperature changes. Second chance
If a hose should rupture or one of the brake lines should become perforated from corrosion resulting in a catastrophic loss of fluid in half the system, the other half will still provide a means of decelerating the vehicle, albeit with a lower pedal and reduced stopping power. Both pistons have extensions which project out in front of their primary seals. A failure in the circuit that is connected to the primary piston’s pressure chamber will allow the piston to move forward enough so the extension will bear on the secondary piston, push it ahead, and generate pressure in the other circuit.
If, on the other hand, the circuit that receives pressure from the secondary chamber springs a leak, the extension on the secondary piston will bottom out on the front of the cylinder and the fluid trapped between the pistons will operate the alternate set of brakes. Extra displacement In the continuing effort by most automakers to wring every last bit of fuel efficiency out of cars, the resistance to rotation that zero-clearance discs cause is unacceptable.
So, low-drag calipers were introduced. These have seal grooves machined at an angle, which cause the seals to retract the pistons enough to eliminate the parasitic loss. But this required a master cylinder that displaced a large volume of fluid during the initial part of the stroke in order to allow normal pedal travel and feel. One common design uses a stepped bore and a primary piston with a small front and a larger rear diameter.
At the beginning of the stroke, the large part of the piston naturally displaces more fluid than the small part, and this extra volume goes around the lip of the small seal into the chamber between the primary and secondary pistons, moving the secondary ahead more than the distance the push rod has traveled. This displaces extra fluid into both circuits. A special valve connected to the rear high-volume chamber vents excess fluid up into the reservoir once a certain amount of pressure is achieved. It also acts as the refill passage for the large chamber when the brakes are released.
