How to Bleed Brakes

by Dave Zeckhausen

How often should you bleed your brakes?

This essential maintenance procedure is frequently overlooked.  Vehicle manufacturers specify brakes should be bled every two years, starting from the date the vehicle was built.  For race cars, it may even be necessary to bleed the brakes before each track event or, if the pedal becomes soft during an event, between sessions.  If you need to bleed the brakes between sessions, it's probably time to switch to a fluid with a higher boiling point, such as Motul RBF660, StopTech STR660, Brembo LCF600, or even Castrol SRF.

Why bleed brakes?

• Fresh brake fluid has a significantly higher boiling point than old fluid, allowing for harder braking without fade.  Brake fluid is hygroscopic and readily absorbs moisture.  The more moisture in the fluid, the lower the boiling point.  That's why you will see two boiling temperatures listed on the side of a container: the "dry" boiling point and the "wet" boiling point. 

• Moisture in the brake fluid promotes corrosion.  Regular bleeding with fresh fluid allows brake components to last longer.  A well maintained brake system can help you avoid ever having to replace calipers, master cylinder or an expensive ABS control unit.

• The bleeding process, done properly, removes air bubbles from the hydraulic system, resulting in firmer brake pedal feel and more linear, responsive braking performance.  Air in the system can be dangerous and result in the pedal sinking to the floor.  Air is compressible, brake fluid is not.

How much brake fluid is required?

It should be possible to bleed all four corners of a car with somewhere between 1/2 and one liter of brake fluid.  If you've just installed new brake lines or a big brake kit, it may require more fluid, especially if you are a novice at bleeding.

Must brakes be bled in a specific order?

Bleeding is done one wheel at a time.  The "old timers" will tell you to start with the wheel furthest from the master cylinder and work your way closer.  Typically, this means RR, LR, RF, then LF.  However, it no longer matters if you start with front or rear wheels, since they are on separate circuits on modern cars with ABS and dynamic stability control systems.

How are brakes bled?

There are many techniques for bleeding brakes and a variety of gadgets to make the job easier.  For simply replacing old fluid with new, many of these techniques work fine.  But for removing air bubbles from the system, one approach is far superior.  That is the "old fashioned method", which requires an assistant to push the brake pedal while the mechanic opens and closes the bleed valve on the calipers.  This 2-person method generates sufficient jolts to the brake fluid to knock loose pesky bubbles and allow them to be flushed away.  Following these instructions will yield a firmer brake pedal, often with better feel than the day the car rolled off the assembly line.

Safety Warning:

Working on your own car can be dangerous.  Even quality jack stands can collapse if not positioned properly, and a floor jack can fail without warning.  You can be injured or killed if you do not follow proper safety procedures.   Zeckhausen Racing LLC assumes no liability, expressed or implied, for the improper use of these instructions.

1. Block a front wheel to prevent the car from rolling.  Raise the back of the car with a floor jack and lower it onto a pair of jackstands.

2. Remove the rear wheels.

3. Open the brake fluid reservoir and remove as much old brake fluid as possible, using a suction tool.  Be careful not to spill any fluid, as it will dissolve the paint on your car.

4. Fill the brake reservoir to the top with fresh brake fluid.

5. Place a box end wrench over the bleed screw on the first rear caliper.  Push a clear plastic tube over the bleed screw nipple and place the opposite end into a catch bottle.  Auto parts stores sell catch bottles with a cover to prevent fluid spills, a 1-way check valve in the cap, and a rubber fitting on the plastic line that fits snugly in place over a wide range of bleed screw sizes.

6. Ask your assistant to push the brake pedal a few times, until it becomes firm.  This is especially important if you have just replaced pads, since it closes the gap between pads and rotors.  If you've installed new brake lines a big brake kit, there may be enough air in the system that the brake pedal will not become firm.  That's OK.  Just move on to the next step.

7. Tap the caliper a few times with a rubber mallet or a "dead-blow" hammer.  This helps knock loose any air bubbles that may be clinging to inside surfaces of the caliper.

8. Have your assistant PUSH hard on the brake pedal and HOLD.  With a quick motion, open the bleed screw about 1/8 turn.  Fluid (and air bubbles) will flow through the clear plastic tube and into the catch bottle.  A shop light, placed behind the plastic tube, will make it easier to see the condition of the fluid coming from the caliper.

9. Close the bleed screw before the brake pedal reaches the floor.  Don't worry if your timing is off at first.  You will quickly figure it out, with feedback from your assistant. 

10. Tell your assistant to RELEASE.

11. Repeat steps 8 - 10.  Do this about 6 times, then stop and check the fluid level in the master cylinder reservoir.  You MUST NOT allow the level to fall to the point where air is sucked into the master cylinder.

12. Top off the brake fluid reservoir and continue bleeding until you no longer see air bubbles coming from the caliper.

13. Repeat the process for the other rear caliper.

14. Ensure both calipers are wiped clean of any brake fluid and that none has spilled on the rotors or pads.  Use brake cleaning spray, if necessary.  Reinstall rear wheels and torque to factory specification, then lower car to the ground.

15. Apply the parking brake, put the car in gear (or PARK) and raise the front of the car with your jack.  Lower it onto a pair of jackstands. 

16. Remove the front wheels.

17. Follow the same steps as with the rear wheels.  The fluid will flow more rapidly from the front calipers, so you should check the level in the reservoir more frequently.

18. Reinstall and torque front wheels, then lower the car to the ground. 

19. Top off the brake fluid reservoir and replace the cap.