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Master Cylinder and Brake Fluid Reservoir Replacement
 
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Pelican Technical Article:

Master Cylinder and Brake Fluid Reservoir Replacement

Time:

30 minutes30 mins

Tab:

$150 to $400

Talent:

*

Tools:

Oil draining hand pump, 11mm & 13mm open end line wrench, 13mm socket wrench, 10 inch socket wrench extension, universal joint for socket wrench

Applicable Models:

Mercedes-Benz R107 (1973-80)

Parts Required:

Master cylinder, brake fluid

Hot Tip:

Use a rag to soak up any extra brake fluid

Performance Gain:

A braking system that will stop your car

Complementary Modification:

Change brake lines

The braking system for your car is arguably the most important function. The braking system is a hydraulic system that employs a fluid pumped through the brake lines to a component that transfers the pressure of the fluid.

In the case of your braking system, the master cylinder is the pump and the fluid is the brake fluid and the device accepting the force of the pumped fluid is the brake pads. The way in which the system works is built upon the principle first explained by Pascal where he states, "When pressure is exerted on any part of a fluid within an enclosed chamber, an equal pressure is transmitted undiminished in all directions and acts with equal force on all equal areas." The common explanation of how hydraulics work is simply a force that is applied at one point is transmitted to another point using an incompressible fluid. That is true to an extent, yet in actuality fluids can be compressed to a small extent. For our purposes let's just say fluids are not compressible and leave it at that.

The thing that is compressible is a gas (or in the case of a brake system) air. Air can get trapped in the fluid and make the brakes feel squishy and unresponsive. The master cylinder can fail due to several issues such as corrosion, contamination or wear. Replacing the brake master cylinder is straightforward and will require new brake fluid to be added to the braking system once the old master cylinder is replaced with a new one.

The M-B 450SL brake system is equipped with a brake pad wear indicator warning that indicates when the pads needs replacing. In 1976, Mercedes began using a step-type master cylinder, which eliminated the use of the vacuum pump that was used on earlier models. The front brakes are connected to the primary side of the master cylinder and account for 75% of the braking, and the rear brakes are activated by the secondary side. There is also a pressure differential indicator used up until 1978 that will light on the dash indicating a partial or total loss of functionality of any part of the brake system. The light will remain on until the reset pin is pressed on the master cylinder. After 1978, the pressure differential warning indicator was eliminated with the step-type master cylinder. This step-type version has two chambers with two sets of electrical contacts. Any loss of fluid in either of the chambers will activate a warning light. The dual master cylinder set-up has a safety feature in which if a leak develops on one side the other side will still function. If the front or the rear fail, the brake-pedal travel will increase drastically.

Also note that because disc brakes are used, there is no need for adjustment. Disc brakes are actually self-adjusting. The hand brake (also known as the emergency brake) may require adjustment. That is a topic for another article.

The master cylinder is located on the left side of the engine and found beneath the brake fluid reservoir.
Figure 1

The master cylinder is located on the left side of the engine and found beneath the brake fluid reservoir. Remember that the left and right of the vehicle are derived from sitting inside the interior of the car and looking forward.  

Using an oil change pump, drain as much of the fluid as possible.
Figure 2

Using an oil change pump, drain as much of the fluid as possible. This will reduce the amount of brake fluid that will spill on the engine and garage floor. Brake fluid can cause paint to soften and possibly peel so care should be taken to make sure none is spilled. I placed the cap back on the brake fluid reservoir in order to reduce the amount of particles that may enter as well as keeping it in a safe spot that will prevent loosing the cap.  

Disconnect both leads that are connected to the brake fluid reservoir.
Figure 3

Disconnect both leads that are connected to the brake fluid reservoir. Often these leads can be loosened by hand and pulled straight out. 

Loosen and remove the first fluid line using an 11mm open end line wrench.
Figure 4

Loosen and remove the first fluid line using an 11mm open end line wrench. Place a rag under the fluid line before you loosen the line in order to absorb residual brake fluid that may leak.

Loosen and remove the other fluid line with an 11mm open-end line wrench.
Figure 5

Loosen and remove the other fluid line with an 11mm open-end line wrench. Place a rag under the fluid line before you loosen the line in order to absorb residual brake fluid that may leak.

Loosen and remove the master cylinder booster nut with a 13mm line wrench that is closest to the side of the car.
Figure 6

Loosen and remove the master cylinder booster nut with a 13mm line wrench that is closest to the side of the car. Place a rag under the fluid line before you loosen the line in order to absorb residual brake fluid that will leak. Replace the nut on the stem so as not to lose it.

Loosen and remove the other master cylinder booster nut with a 13mm socket fixed to a universal joint and a 10 inch extension socket wrench.
Figure 7

Loosen and remove the other master cylinder booster nut with a 13mm socket fixed to a universal joint and a 10 inch extension socket wrench. Replace the nut on the stem so as not to lose it. I could not figure a better way to remove this bolt without removing a host of other hardware, which would only increase assembly time as well as frustration. If I meet the person who invented a universal joint and or an extension to a socket wrench I will surely buy them a beer!

The master cylinder and brake fluid reservoir are now free.
Figure 8

The master cylinder and brake fluid reservoir are now free. Be careful not to spill the brake fluid. 

In order to remove the brake fluid reservoir from the master cylinder, it is easiest to set it into a vice.
Figure 9

In order to remove the brake fluid reservoir from the master cylinder, it is easiest to set it into a vice. 

The reservoir can be pulled off the master cylinder with slight effort.
Figure 10

The reservoir can be pulled off the master cylinder with slight effort. The master cylinder is now free to be rebuilt or replaced. The brake fluid reservoir can also be replaced now as well. Installation is the reverse of removal. Please refer to the Adding Brake Fluid and Bleeding Brake Lines articles for charging the lines and system with new brake fluid. 



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Page last updated: Wed 12/7/2016 03:06:58 AM