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Pelican Technical Article:

Bleeding Brake Lines

Time:

10 min

Tab:

$0 to $50

Talent:

*

Tools:

9mm open ended wrench, an extra foot, Mityvac oil vacuum pump (optional)

Applicable Models:

Mercedes-Benz R107 (1972-80)

Parts Required:

Break fluid

Hot Tip:

Use rags to absorb spilled brake fluid

Performance Gain:

A braking system that will stop your car

Complementary Modification:

Change brake pads

It is very probable that your brakes will feel squishy at some point. This is most likely due to having trapped air in the brake fluid. While brake fluid is not all that compressible, air voids are compressible. The brakes work by applying pressure to the rotors by pumping brake fluid to the brake pads. It's a means to transfer power by way of using the fluid pressure. Keep in mind with pressure there is heat, both on the pads but also the fluid. The brake rotors and pads get very hot and the heat is transferred to the brake fluid as well. If the brake fluid gets too hot and boils it effectively puts air in the lines and will lead to failure. The DOT designation for the fluid refers to the chemistry of the fluid. The DOT standards refer to a brake fluid's "dry" and "wet" boiling points. The wet boiling point, which is usually much lower (although above most normal service temperatures), this refers to the fluid's boiling point after absorbing a certain amount of moisture. This is several (single digit) percent points varying from formulation to formulation. Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic which means they absorb water absorbing from the atmosphere under normal humidity levels. Non-hygroscopic fluids (e.g. silicone/DOT 5-based formulations), are hydrophobic which means they typically won't absorb water readily and can maintain an acceptable boiling point over the fluid's service life. These fluids are susceptible to potential phase separation, water pooling and freezing or boiling in the system over time.

 DOT Standard Dry Boiling Point Wet Boiling Point Chemistry
 DOT 2 374 degrees F 284 degrees F Caster Oil
 DOT 3 401 degrees F 284 degrees F Glycol Ether
 DOT 4 446 degrees F 311 degrees F Glycol Ether Borate Ester
 DOT 5 500 degrees F 356 degrees F Silicone
 DOT 5.1 500 degrees F 356 degrees F Glycol Ether Borate Ester

As for fluid compatibility, you can put DOT 4 in place of Dot 3 but not the other way around. DOT 5 is completely different and is silicone based, it is not compatible with DOT 3 and 4 systems. They don't mix well at all therefor DOT 5 is not interchangeable or compatible with DOT 3, 4, and 5.1 fluids and can cause catastrophic system failure.

The Dot 3, 4, and 5.1 are glycol ether based. They are compatible, but like motor oils, you should use the recommended the higher grade fluid. The Dot 4 and 5.1 also have borate ester to handle higher temperatures. The DOT 3, 4, and 5.1 fluids are found in most brake and clutch systems.

The DOT 5 is a silicone oil based fluid and can only be used in new, dry systems. It is found in racing, US Military, Soviet, and Finnish systems to handle low temperature, excessive braking, and water exposure problems. Consult the material compatibility of your fluids before charging your system. While the DOT 5 may appear advantageous if you race, if may have material compatibility issues with lines and components.

The viscosity of DOT 4 is more than DOT 3 and it maintains its fluidity at a higher temp. Also, the brakes shall be more effective when the system gets hot during a long drive.

To properly bleed the lines of any trapped air, new fluid should be added to the system. To do this you will need to start by locating the brake fluid reservoir.

The brake fluid reservoir is located under the hood, on the driver's side.
Figure 1

The brake fluid reservoir is located under the hood, on the driver's side.

You should have someone help you out.
Figure 2

You should have someone help you out. They will need to pump the brake pedal to the floor when instructed.  

Remove the bleeder screw cap located on the brake caliper.
Figure 3

Remove the bleeder screw cap located on the brake caliper. You may also find it easier to work on the brakes if you turn the wheel so that the front is pointing outward. This allows greater access to the brakes. In the picture you will notice that the wheel is positioned to make a right turn.    

if you don't have a Mityvac® oil vacuum pump don't sweat it.
Figure 4

If you don't have a Mityvac® oil vacuum pump don't sweat it. You can still bleed the brakes. Instead of hooking up the Mityvac® to the bleeder screw, simply use a rubber tube found at the hardware store and make sure you allow the brake fluid that will be bled off to do so into the container. You shouldn't reuse the fluid but simply dispose of it according to local and federal regulations. Connect the bleed tube assembly to the Mityvac®.

Connect compressed air to the quick disconnect nipple to the swivel air inlet located near the handle.
Figure 5

Connect compressed air to the quick disconnect nipple to the swivel air inlet located near the handle. The system is now ready to use.

Attach the bleed screw adaptor to the bleed screw.
Figure 6

Attach the bleed screw adaptor to the bleed screw.

Using a 9mm open end line wrench, turn the bleed screw clockwise to open the flow of brake fluid.
Figure 7

Using a 9mm open end line wrench, turn the bleed screw clockwise to open the flow of brake fluid. Try not to spill any because it is corrosive as well as toxic to animals.

There is a lever on the handle which you press forward to activate the vacuum action.
Figure 8

There is a lever on the handle which you press forward to activate the vacuum action. Push forward to the 'ON' position. You should hear a sucking sound. If you don't check to see that you have the compressed air turned on. 

Bleed the brake line using the Mityvac®: start by connecting the bleed screw adaptor on to the bleed tube assembly then loosen the screw using a 9mm open end wrench while having someone else pump the brakes.
Figure 9

Bleed the brake line using the Mityvac®: start by connecting the bleed screw adaptor on to the bleed tube assembly then loosen the screw using a 9mm open end wrench while having someone else pump the brakes. If you're using the Mityvac®, it should begin to fill with brake fluid. If you are just using a tube then you may notice the fluid is foamy. 

Instruct the person in the driver's seat to press the brake pedal. When the brake pedal hits the floor, close the bleed screw before allowing the pedal to return. This will prevent air from entering back into the system. If the flow doesn't appear strong you may want to open up the brake fluid reservoir cap to allow for greater flow. 

You will have to add brake fluid to the reservoir after each wheel you bleed. Repeat these same steps for all wheels.  


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Page last updated: Wed 12/7/2016 03:06:57 AM