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Pelican Technical Article:

Replacing Brake Fluid

Time:

30 min

Tab:

$50 to $100

Talent:

**

Tools:

Brake Bleeder

Applicable Models:

Mercedes-Benz R107 (1973-80)

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

The brake system is made up of a caliper which holds the brake pads. The brake pads press against the brake rotors and slow the rotation of the wheels. The master cylinder pumps the brake fluid to actuate the pads applying pressure to the rotors. Over time the brake pads wear down. There are sensors which send a signal indicating wear to the point of replacement. Replacing the brake fluid is straight forward.

When changing brake fluid consider the following. Your brakes work by applying pressure to the rotors. With pressure there is heat. 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 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, it 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 will be more effective when the system gets hot during a long drive.

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.

Using the Mityvac® or a hand held vacuum oil pump, remove as much of the brake fluid as possible.
Figure 2

Using the Mityvac® or a hand held vacuum oil pump, remove as much of the brake fluid as possible. Try not to spill any because it is corrosive as well as toxic to animals.

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

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 wrench, turn the bleed screw clockwise to open the flow of brake fluid.
Figure 7

Using a 9mm open end 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 compressed air turned on. 

Bleed the brake line using the Mityvac®.
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 line wrench while having someone else pump the brakes. If you're using the Mityvac®, it should begin to fill with brake fluid. Empty all of the fluid from the lines. Repeat for all the wheels. 

When the brake pedal hits the floor, close the bleed screw before allowing the pedal to return.
Figure 10

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. 

Now that the system is void of fluid, turn off the vacuum and with the Mityvac® still attached pour new fluid into the reservoir. As soon as the reservoir is at the full line draw fluid into the Mityvac® again. This will draw any trapped air. Repeat this process two to three times with all wheels. 

When you finish charging the system, turn the ignition and start the car.
Figure 11

When you finish charging the system, turn the ignition and start the car. Pump the brakes several times. You should feel the brakes tightening up. You may have to repeat the bleeding procedure a few times before you notice that there are no air pockets and the brakes feel tight.  

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Page last updated: Fri 12/9/2016 03:05:29 AM