When it comes to brakes, you need think of a complete system consisting of components working together to stop your vehicle. As brake systems evolved from 4 wheel drum systems to rear drum/front disc combinations, the design of the various components making up the system changed in order to keep pace with the new technology. One of the best examples of this is illustrated by the changes in master cylinder design.
Outside of looks, the main differences between master cylinders from earlier eras and today are single versus dual reservoirs. Dual reservoirs came about simply from a safety standpoint. A single reservoir master cylinder is providing pressure to both the front and rear systems. Should a failure exist or happen in the master cylinder, there is a very good chance that all brakes will be lost in the vehicle. With a dual reservoir system that splits the car into front and rear, you stand a chance should a failure occur.
When dual reservoir master cylinders originally made it onto the scene, one of the reservoirs was much larger than the other. This was generally considered the reservoir for the drum brakes due to the sheer volume of fluid needed to push the brake shoes out followed by the return of all that fluid back to the master cylinder. Disc brakes didn’t require the same volume in the reservoir. Today, most master cylinders have the same size reservoir as well as masters sharing the same reservoir. These are still considered dual reservoir systems because the internals are separated front to rear.
When using a dual reservoir master with disc brakes on the front, the output pressure is equal on each port and must be regulated or adjusted through an external valve to provide proper balance. When using a dual reservoir master cylinder along with four wheel drums, a valve is not required or needed as you want equal pressure from the master cylinder and the different bore size in the wheel cylinders from front to rear does the proportioning. Bore size in the master cylinder is important in any scenario. Remember, bore size is directly related to the rest of the brake system. The best way to understand brake pressure when choosing a master cylinder is to think of a garden hose. When putting your thumb over the end of the hose, you create more pressure. At the same time, you don’t want to run out of water. With smaller bore sizes, we create more pressure. Make sure to have enough brake fluid in the master to service the calipers at the correct pressure.
Speaking of pressure, if you have spent some time on our site, you probably noticed that we offer master cylinder / brake booster combos and you may wonder why. The short answer is when replacing or upgrading the master, adding in the booster will give you much better pedal feel. The assist that a booster provides allows less pressure to be applied to the pedal while maintaining the proper brake pressure needed in the system.
If you want to learn more about master cylinders, give us a call or check out our Master Cylinder FAQ. Here you will find answers to common questions regarding this important brake system component.