Braking Systems 101

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How To Replace Your Master Cylinder

DIY: How To Replace/Rebuild A Brake Master Cylinder
The brake master cylinder, indeed the whole brake system, is probably the most important part of your vehicle. It is a fairly easy job to replace or rebuild and something a beginner can easily do.

What you will need:

  1. Open end or box wrenches
  2. Line wrenches
  3. Screwdrivers
  4. Small pry bar
  5. Snap ring pliers
  6. New or Rebuilt master cylinder or...
  7. Master cylinder rebuild kit
  8. Fender covers
  9. Brake cleaner
  10. Fresh brake fluid
  11. Brake lube
  12. Safety glasses
  13. Shop towels
  14. Pail of clean water
  15. Latex Gloves (Optional)
Before You Start:
  • Follow these instructions carefully. Read and be sure you understand them before you begin.
  • Gather together all of your tools and supplies before you begin.
  • Allow plenty of time to do the job so you don't have to hurry.
  • Remember that these are general instructions. For more detailed instructions pertaining to your specific vehicle, consult an appropriate repair manual.
  • Beware of hot objects, sharp instruments and hazardous materials.
  • Don't substitute tools unless you're sure you won't compromise either your safety or the performance of your vehicle.
  • Never work on a vehicle that is only supported by a jack. Use jack stands to support the vehicle while you work. Work on a solid, level surface. Never jack a car up on dirt or grass.
  • Check for any brake fluid leaks or cracked brake lines. Replace them as necessary.

The Brake Master Cylinder
The brakes are one area where you do not want to cut corners. Any brake work must be done properly to maintain maximum brake efficiency. If you feel you are not up to the task, take it to a professional mechanic.
The brake master cylinder, indeed the whole brake system, is probably the most important part of your vehicle. As I have always said, there is no sense in making the car go if you can't make it stop. As logical as this sounds, I have had customers come in and forego brake work in favor of improving their stereo systems or fancy wheels and tires. They probably want to listen to music and look good in a wreck while they wait for the ambulance to come and take them to the hospital.
Most modern car braking systems are broken into two circuits, with two wheels on each circuit to increase safety. In most vehicles it is one front wheel and one rear wheel although some vehicles split front and rear wheels. With a dual system like this, if a brake fluid leak occurs in one circuit, you only lose 50% of your brakes. You will be able to stop although it will take somewhat longer.
The master cylinder supplies pressure to both circuits of the car. It uses two pistons in the same cylinder in a way that supplies brake pressure to both circuits and it is very reliable. There is a combination valve that warns the driver, among other things, if there is a problem, such as a leak, with the brake system.
The brake system uses a special fluid to operate the system. Brake fluid is a specially formulated liquid used in the brake hydraulic system. Brake fluid must meet one of three U.S. Department of Transportation (DOT) specifications. DOT 3 and DOT 4 are glycol-based fluids, which absorb water. DOT 5 is a silicone-based fluid and does not absorb water. The main difference is that DOT 3 and DOT 4 absorb water, while DOT 5 doesn't. Most cars use DOT 3 fluid from the factory.
Okay, let's get to work!


Removing the master cylinder is really simple, only requiring a few steps. Keep in mind that brake fluid will eat the paint of the car. So use a plastic or vinyl type fender cover to protect the vehicle's paint. Brake fluid is water soluble so if you should get some on the paint, wash it off with plenty of water as quickly as possible.

  1. Using a siphon, an old turkey baster works great, empty the master cylinder reservoir and dispose of the old brake fluid in the proper manner.
  2. Disconnect the wire connector for the brake fluid level and/or brake pressure-warning switch, if your vehicle is so equipped.
  3. Disconnect the brake lines from the master cylinder with a line wrench. A line wrench is specially designed to remove and install hydraluic fittings without rounding them off.
  4. Remove the master cylinder mounting nuts, then the master cylinder.
  5. On non-power brake vehicles, disconnect the master cylinder linkage from the brake pedal underneath the dash.
  6. On models with separate fluid reservoirs, remove the reservoir.
Before installing the new master cylinder on the vehicle, it must first be bled using a specialized bleeder kit. The master-cylinder bleeder kit comes with tubing, clips and multiple adapters. Most new or rebuilt master cylinders come with a bleeder kit for that particular master cylinder.

  1. Begin by removing the cap.
  2. Follow the directions that come with the kit to select the suitable adapters, and then connect the adapters and tubes to the ports on the cylinder. The other ends of the tube extend down into the master cylinder, and are held in place by a plastic clip. Figure 2.
  3. With the bleeder kit installed, fill the master cylinder about halfway with new brake fluid. Use a large screwdriver to depress the valve assembly inside the master cylinder. If you do not have a vise you can mount the master cylinder on the vacuum brake booster and bleed the master cylinder.
  4. Small bubbles will appear in the fluid. Continue working the valve assembly until no more bubbles appear. This indicates that the cylinder has been thoroughly bled.

Fig. 2. Bench Bleeding The Master Cylinder
  1. Reverse the removal procedure to install the master cylinder, noting the following:
    • Refer to your service manual for any Master Cylinder Push Rod Adjustment procedures if your vehicle requires it.
  2. Bleed brakes as described under Brake Bleeding.
  3. Operate the brakes several times and check for external hydraulic leaks.
This is fairly straightforward job. Replacing a master cylinder should take you about 1 to 2 hours, depending on the make and model of car. If you are rebuilding the master cylinder, add about another hour or so.

Rebuilding The Master Cylinder
Rebuilding a brake master cylinder is not difficult and doesn't require any special tools. Nine times out of ten a master cylinder goes bad because it has an internally leaking seal or cup. Replacing these seals and cups will bring the master cylinder back to life.
Refer to figures 3 and 4 when performing the following procedures.

  1. Remove the master cylinder from the vehicle as follows:
  2. Disconnect and plug the hydraulic lines.
  3. Remove the two master cylinder-attaching nuts, then the master cylinder.
  4. Remove the reservoir cover and diaphragm. Discard the old brake fluid.
  5. Inspect the cover and diaphragm. Replace if it is cut, cracked or deformed.
  6. Remove the fluid level switch, if equipped.
  7. Depress the primary piston and remove the lock ring.
  8. Plug primary fluid outlet, outlet nearest to the cowl when master cylinder is installed, then apply compressed air into the secondary fluid outlet to remove the primary and secondary pistons.
  9. Remove the spring retainer and seals from the secondary piston.
  10. Clamp the master cylinder in a vise and remove the reservoir using a pry bar. Remove the reservoir grommets.
  11. Inspect the master cylinder bore for corrosion. Do not use abrasive material on master cylinder bore. Replace if the bore is corroded.

Fig. 3. Internal Parts Of The Master CylinderAssembly:

  1. Clean all parts not included in the repair kit with brake fluid. Do not dry with compressed air. Lubricate all the rubber parts with clean brake fluid prior to installation.
  2. Lubricate the new reservoir grommets with silicone brake lube, then press the grommets into the master cylinder body. Ensure the grommets are properly seated.
  3. Lay the reservoir upside down on a flat, hard surface. Press the master cylinder body onto the reservoir using a rocking motion.
  4. Install new seals on the secondary piston, then the spring retainer.
  5. Install the spring and secondary piston assembly into the cylinder.
  6. Install the primary piston. Depress the primary piston into the cylinder and then install the lock ring.
  7. Install the fluid level switch, if equipped.
  8. Fit the diaphragm into the reservoir cover, and then install the cover onto the reservoir.
  9. Install the master cylinder and bleed the brake system.

Fig. 4. Internal Parts Of The Master Cylinder

Bleeding The Brake System

NOTE: Pressure bleeding is recommended for all hydraulic systems. However, if a pressure bleeder is unavailable, use the following procedure. Some hydraulic systems require vacuum bleeding. Check your vehicles service manual for the procedure.
Refer to figures 3 and 4 when performing the following procedures.
CAUTION: Brake fluid damages painted surfaces. Immediately clean any spilled fluid.

  1. Remove the vacuum reserve by pumping the brakes several times with the engine off.
  2. Fill the master cylinder reservoir with clean brake fluid. Check the fluid level often during bleeding procedure; do not let the reservoir fall below half full.
  3. If necessary, bleed the master cylinder as follows:
    • Disconnect the master cylinder forward brake line connection until fluid flows from the reservoir. Reconnect and tighten the brake line.
    • Instruct an assistant to slowly depress the brake pedal one time and hold.
    • Crack open the front brake line connection again, purging air from the cylinder.
    • Retighten the connection and slowly release the brake pedal.
    • Wait 15 seconds and then repeat until all the air is purged.
    • Bleed the rearward brake line connection by repeating the preceding steps.
    • Loosen and then slightly retighten the bleeder valves at all four wheels. Repair any broken, stripped or frozen valves at this time.

Fig. 5. Bleeding The Brake System
  1. Proceed to the appropriate wheel first and follow the set sequence according to the Wheel Bleed Sequence.
  2. Place a transparent tube over the bleeder valve and then allow the tube to hang down into a transparent container, figure 5. Ensure the end of the tube is submerged in clean brake fluid.
  3. Instruct an assistant to slowly depress the brake pedal one time and hold.
  4. Crack open the bleeder valve, purging air from cylinder. Retighten the bleeder screw and slowly release pedal.
  5. Wait 15 seconds and then repeat the preceding bleed steps. Repeat these steps until all the air is bled from the system.
Wheel Bleed Sequence:
If you are bleeding the brakes manually, the sequence is RR-LR-RF-LF. If you are pressure bleeding, bleed the front brakes together and the rear brakes together.
Courtesy Auto Repair