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Audi A4 Brake Master Cylinder and Resevoir Replacement
 
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Pelican Technical Article:

Audi A4 Brake Master Cylinder and Resevoir Replacement

Steve Vernon

Time:

3 hours3 hrs

Tab:

$150

Talent:

**

Tools:

T20 Torx, 22mm wrench, 13mm flare nut wrenches, power bleeder, Audi scan tool

Applicable Models:

Audi A4 (2002-05)
Audi A4 Cabriolet (2003-06)

Parts Required:

New master cylinder, reservoir, brake fluid

Hot Tip:

Use paper towels and plastic bags to contain any fluid leaks

Performance Gain:

Better braking

Complementary Modification:

Replace brake booster

Without a doubt, your brakes are the most important system on the car. The heart of the brake system is the master cylinder, which controls the hydraulic pressure of the entire system. Unfortunately, over many years, the master cylinder and reservoir have a tendency to wear out and leak. The leakage can occur internally or externally, resulting in a weakened braking system. They also can get damaged by water in the system if you do not change the fluid often enough. Brake fluid is hydroscopic, which means it absorbs water. Once the fluid has absorbed a certain percentage of moisture (usually around 2%) it must be changed. Water in the system can cause both corrosion to the brake system as well as seriously degrade the performance or stopping power of your brakes.

If you have any problems with your brakes, and you think that it's related to the master cylinder or reservoir, you should check both of them and replace as needed.

Replacing the master cylinder on the Audi A4 is not difficult, it is just a tight fit, but it should take no more than three hours including bleeding the brakes. Note; while people have claimed to be able to remove the master cylinder from the vehicle without removing the booster it was not physically possible on our project car. This article will show you every step you need to remove the master but, if like us you cannot get it out from the tight space and need to remove the booster as well, you will need to see our article on removing your brake booster to complete the work.

When you are finished you will need to completely bleed the brakes. DO NOT drive the car without completely bleeding the brakes. You will need the Audi scan tool to put the ABS system into bleed mode.

Begin by removing the cowl and battery cover from area just below the windshield; it just pulls off (red arrow).
Figure 1

Begin by removing the cowl and battery cover from area just below the windshield; it just pulls off (red arrow).

With the cover off you can see the reservoir and cap (green arrow) and where the master cylinder attaches to the brake lines ((red arrows).
Figure 2

With the cover off you can see the reservoir and cap (green arrow) and where the master cylinder attaches to the brake lines ((red arrows). If you have a manual transmission there will be a line coming out of the left side of the reservoir to feed the clutch hydraulics (yellow arrow).

Begin by sucking out as much fluid as you can.
Figure 3

Begin by sucking out as much fluid as you can. Use a turkey baster or fluid pump and suck as much fluid as possible out (red arrow). You will not be able to get all the fluid out so be prepared for some spillage. Remember if you get any fluid on your hands, gloves or clothes and touch your paint you will quickly ruin your paint job. Use caution and work carefully.

There is a long screw that holds the reservoir to the master cylinder.
Figure 4

There is a long screw that holds the reservoir to the master cylinder. It is located below the reservoir and can be difficult to get at. Our car needed a T20 Torx to remove it (red arrow).

Use care when removing the screw as you do not want to drop it down into the vehicle (red arrow).
Figure 5

Use care when removing the screw as you do not want to drop it down into the vehicle (red arrow).

Before removing the reservoir clean around the master cylinder, especially if you are only replacing the reservoir.
Figure 6

Before removing the reservoir clean around the master cylinder, especially if you are only replacing the reservoir. You do not want any dirt or debris getting into the master (red arrow). Gently pry the reservoir off of the master cylinder, it is only held in place by the two plastic nubs on the bottom of the reservoir sitting in the rubber grommets in the master. Wiggle and pry it up and it will pop out. You will need to remove the brake fluid level sensor (yellow arrow) before remove the reservoir from the vehicle.

If you are only replacing the reservoir make sure to replace the grommets as well (red arrow).
Figure 7

If you are only replacing the reservoir make sure to replace the grommets as well (red arrow). It is a good idea to cover the openings into the master at all times to make sure nothing can get in and destroy the master. If you are just replacing the reservoir from this point installation is the reverse of removal. Remember you cannot drive the vehicle until you have completely bled the system.

If you are replacing the master cylinder you will need a 13mm flared nut wrench; this wrench will grab 5 sides of the nut and prevent stripping.
Figure 8

If you are replacing the master cylinder you will need a 13mm flared nut wrench; this wrench will grab 5 sides of the nut and prevent stripping. If you round off a nut on a brake line you will need to replace the entire line as the nuts cannot be replaced separately. Be smart and use the right tool for the job.

Remove the two lines coming out from the master cylinder (red arrows).
Figure 9

Remove the two lines coming out from the master cylinder (red arrows).

If you are replacing the master and especially if you are also replacing the booster it is a good idea to completely remove the brake lines from the master cylinder to the ABS unit.
Figure 10

If you are replacing the master and especially if you are also replacing the booster it is a good idea to completely remove the brake lines from the master cylinder to the ABS unit. You are going to need to bleed the system when you are done so removing the lines will give you a lot more room to work and eliminate the possibility of damaging them. Clean around the lines and then use your 13mm flared nut wrench and remove the lines from the top of the ABS control unit (red arrows).

If you remove the ABS control unit brake lines make sure you have properly cleaned around the top and then plug the openings so no dirt or foreign matter can get into the unit (red arrows).
Figure 11

If you remove the ABS control unit brake lines make sure you have properly cleaned around the top and then plug the openings so no dirt or foreign matter can get into the unit (red arrows).

The lines are protected by a rubber grommet where they run through the sheet metal; remove the grommet with the lines (red arrow).
Figure 12

The lines are protected by a rubber grommet where they run through the sheet metal; remove the grommet with the lines (red arrow).

The master cylinder is attached to the booster by an E10 Torx bolt (red arrow) inside of the 22mm nut (yellow arrow).
Figure 13

The master cylinder is attached to the booster by an E10 Torx bolt (red arrow) inside of the 22mm nut (yellow arrow). The nut holds the master cylinder to the booster while the E10 Torx bolt holds the booster to the firewall.

You can remove the 22mm nut (red arrow) freeing the master cylinder while still leaving the booster attached to the firewall by the E10 Torx bolt (yellow arrow).
Figure 14

You can remove the 22mm nut (red arrow) freeing the master cylinder while still leaving the booster attached to the firewall by the E10 Torx bolt (yellow arrow).

The master cylinder is now free from the booster and can be removed from the vehicle.
Figure 15

The master cylinder is now free from the booster and can be removed from the vehicle. Installation is the reverse of removal. While I have heard of people removing the master without removing the booster on some cars it was not possible to do it without the risk of damaging things. If you cannot get your master out please do what we did and remove the brake booster and master cylinder. Please see our article on brake booster replacement for further instruction. Make sure when installing the master cylinder that the push rod is installed correctly in the master cylinder and always replace the rubber sealing O-ring on the master (red arrow). Reinstall everything and bleed the brakes. DO NOT drive your car until you have completely bled your brakes! You will need the Audi scan tool to put the ABS system into bleed mode.



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