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Master Cylinder Q&A

Ah, the master cylinder. One of the many components most of us don't give a second thought to—until it either goes bad or we have to choose one for a street rod or race car. Then that master cylinder seems pretty darn important.

When the time comes to ponder your master cylinder, you'll be glad you read these Q & As now. As the saying goes, a little knowledge can go a long way—in this case, a long way to stopping you in time.

Q: How does a master cylinder work?
A: The master cylinder supplies hydraulic pressure to the braking system. When you first touch the brake pedal on a modern dual circuit braking system, linkage from the pedal pushes on the primary circuit piston in the master cylinder, building pressure in the system. As you further depress the pedal, more pressure builds and forces the secondary circuit piston to compress its fluid. If the brakes are operating properly, the pressure will be the same in both circuits. That allows the brakes to operate smoothly.

Q: How can I tell if a master cylinder is bad?
A: Generally speaking, an excessively spongy (soft) brake pedal, or a pedal that slowly sinks to the floor as you keep pushing on it, means the master cylinder is leaking and requires replacement. In a dual circuit master cylinder, lost pressure between the primary and secondary circuits due to leakage causes the primary piston to contact the secondary piston. The secondary circuit will function normally, but you will have brakes on just two wheels.

Q: Why should I replace my musclecar's single-reservoir master cylinder with a dual-reservoir cylinder?
A: One word: safety. The master cylinder supplies hydraulic pressure to your vehicle's braking system. Up until the mid-1960s, master cylinders were a single-circuit type that served both the front and rear brakes. If the cylinder failed, so did your brakes. Most vehicles built since 1967 have dual-circuit master cylinders as required by U.S. law. Each circuit controls a pair of brakes (either front, rear, or a diagonal pair); if one circuit fails, you still have enough braking power to slow down in a controlled manner.

Q: What is the difference between a disc brake and a drum brake master cylinder?
A: Disc brakes require more fluid to operate, so a disc brake master cylinder holds a larger volume of fluid than a drum brake master cylinder. A four-wheel drum brake cylinder has a residual pressure valve to maintain a small amount of pressure on the brakes. This prevents air from entering the braking system. Using a drum brake master cylinder with disc brakes reduces braking effectiveness due to decreased fluid volume and rotor “drag” caused by the pressure retained by the residual valve.

Q: What is the difference between a manual brake and a power brake master cylinder?
A: Manual brake master cylinders have a smaller piston bore diameter to maintain sufficient fluid pressure in the braking system. They also have a deeper hole for the brake pedal pushrod. You can use a manual brake cylinder on a power brake system, but not vice-versa; the power brake cylinder's larger piston bore cannot supply the required level of hydraulic pressure required by manual brakes.

Q: Can I use a master cylinder from a disc/drum brake system with a four-wheel disc brake system?
A: No. A master cylinder designed for four-wheel disc brakes is required to supply the proper fluid pressure and volume.

Q: What is the proper master cylinder bore size for manual brakes?
A: The bore should be no larger than one inch to maintain the proper level of hydraulic pressure.

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