What Is a Brake Master Cylinder?
A brake master cylinder is basically a hydraulic pump actuated by the brake pedal and pressure applied to said pedal by the driver’s foot. Brake fluid is the hydraulic fluid used as its driving force under the basic principles of hydraulics. It uses a piston within the brake master cylinder to pump and maintain fluid pressure through the brake lines to the brake components to aid in slowing/stopping the vehicle. For disc brakes it is the brake calipers the fluid is routed to that provides clamping force on a vehicle’s brake rotors to slow down. If you have a drum brake system, that fluid pressure is applied to the wheel cylinders, which applies outward pressure to the brake shoes on the inner lining of the brake drum to provide the stopping power.
Where Is the Master Cylinder Located?
In most instances, the brake master cylinder is in the engine compartment. However, in many applications where engine compartment space is at a premium, like hot rods and race cars, the brake master cylinder could be under the dash or even under the floor. Regardless, the brake master cylinder is connected to the brake pedal in some fashion. The brake master cylinder receives its pressure input from the brake pedal.
If you have a floor-mounted brake pedal, chances are the brake master cylinder is under the floor between the pedals and the driver seat. These systems sometimes have a trap door to access the master cylinder with a reservoir. Some race-oriented brake master cylinders feature a remote master cylinder reservoir mounted to a roll bar or in the engine compartment. A master cylinder with reservoir that is remotely mounted often allows a more compact pedal/master cylinder arrangement under dash or floor with the convenience of the reservoir being easily accessed under hood.
Are All Master Cylinders the Same?
All master cylinders perform the same function. A master cylinder for brakes is specifically designed to apply brake fluid under pressure to each caliper or wheel cylinder. The one area where brake master cylinders differ is in the direction the fluid goes once it leaves the master cylinder. Sometimes referred to as circuits, early brake master cylinders were of the single reservoir variety (sometimes referred to as a “jelly jar” master cylinder). The master cylinder had just one reservoir, or circuit, providing pressure to all four wheels. This was fine until a brake line or hose failure, which with that arrangement means all braking is lost.
With a dual master cylinder, there are two reservoirs, or circuits, responsible for feeding brake fluid to all four corners. Most often, one circuit is responsible for feeding the front brakes, while the other feeds fluid to the rear brakes. In a dual master cylinder arrangement, you can lose a rear brake hose, yet still have adequate pressure to the front brakes to bring the vehicle to a stop.
How Can I Tell When the Master Cylinder Is Bad?
The most obvious sign of a possible bad brake master cylinder is the fact your brakes are under performing. Other signs pointing to a possible bad brake master cylinder is a spongy brake pedal, a brake pedal that goes to the floor, or a brake pedal that doesn’t maintain pressure. Brake master cylinders can last longer than the car they’re bolted to because for the most part they’re sealed from any outside elements. However, if anything gets into the braking system, those contaminants can infiltrate the master cylinder and cause it to become ineffective. All the above brake pedal signs are either a fluid leak or a defective master cylinder. Therefore, by bleeding the system, and then checking the system for leaks, if none appear, a closer look at the brake master cylinder is in order.
Can I Drive with A Bad Master Cylinder?
Well, we don’t recommend it, but we’re sure your dad once had to use both feet to stop his old Thunderbird, which we don’t recommend either. Obviously, it’s unsafe to drive a car with a compromised braking system, and if your vehicle’s braking system is exhibiting any of the above characteristics, replacing it is of utmost importance. Frankly, it is just not worth the risk trying to use the parking brake to get home, or continually pumping the pedal hoping to have enough pedal pressure to stop your car (until you run out of fluid that is). It is much safer and cheaper to pay the tow bill home or to your favorite repair garage than to pay for the damage caused by running into someone while trying to nurse your car home.
What, If Any, Is the Difference Between a Power Brake Master Cylinder and A Manual Brake Master Cylinder?
Generally, a manual brake master cylinder will have a smaller piston bore, or cylinder, compared to a power brake master cylinder. A power brake master cylinder has the help of power assist to aid in getting brake fluid to the brakes so the bore can be larger. Most manual brake master cylinder applications boast a 1-inch bore, while many power brake master cylinders feature a 1-1/8-inch bore. Manual brake master cylinders can also feature smaller lines and fittings as opposed to power brake master cylinders. Application specific manual and power brake master cylinders do differ, mainly in the pedal pin bore depth (power brake masters use a shallow pin depth to mate the power brake booster), so confirm you are ordering the correct master cylinder. However, many brake master cylinders, whether power or manual, are compatible with either system when the proper adapter is provided. You will see on our website brake master cylinders that are compatible with both power and manual brakes and include the machined adapter for the pedal pin bore. Our site has a bevy of options from which to choose, both power and manual.
