difficulty: 2/5
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Disclaimer: Before you attempt any brake work on your car, refer to the factory service manual and follow all precautions. Any and all information presented on this website is not a substitute for a certified professional mechanic. See the TOS Agreement for the full legal disclaimer.
Warning: incorrect work on the brakes could cause a total loss of braking. Verify all of your work by the procedures in the factory service manual before attempting to drive the car. If there is any error or doubt on the work, tow it to a professional mechanic!
Introduction
When you press the brake pedal down, it moves a rod which moves the piston inside the brake master cylinder. That piston moves brake hydraulic fluid which moves a piston in the brake calipers. That is what clamps the brake rotors. The force of the pedal is assisted by the vacuum brake booster, the big round black disk that is under the master cylinder.
The master cylinder has 2 lines coming out of it because it separates the front and rear circuits. If one circuit fails, you might be able to pump the brake pedal to build up pressure in the other circuit.
If you do experience a loss of braking action, some of your priorities should
be to keep control of the car, pump the brakes and brake as needed, steer the
car towards a safe spot, and apply the parking brake to slow down. The
parking brake cable on your Passat actuates the rear calipers with a lever at
the caliper. It's not as effective as using the brake pedal but it can
help slow down the car. You can try downshifting to use engine braking to slow down the
car as well. Do not get distracted from driving when trying to
troubleshoot a braking problem, your first priority is to keep control of the
car! If you see leaking brake fluid around the wheels or in the footwell
near the brake pedal or master cylinder, have the car repaired before driving!
If you have no braking authority and the pedal goes straight to the floor or
almost to the floor, the car has lost brake fluid pressure. Pump the
brakes and if either the front or rear brake fluid circuit is still good, you
should get some braking action on the good circuit. Also apply the parking
brake as needed. A common cause of this is a bad brake line near the
caliper. Another possible cause is air contamination. If you are in
a racing environment, the brakes were overworked and you probably overheated the
pads/rotors or boiled the brake fluid.
If you have no braking authority but you feel resistance in the pedal, pump the
brakes and try to build up pressure and apply the parking brake as needed.
Possible causes are bad caliper, master cylinder, or brake pads. If the brake
slowly goes to the floor, this is normally a bad master cylinder. Look
under the dashboard and at the engine bay firewall for leaking fluid or stains.
If the brake pedal feels very stiff and hard but you still have braking
authority, just press very hard on the pedal. This is normally a faulty
brake booster caused by a vacuum leak.
See 1000q: brake FAQ for more, info, mythbusting, and general tips on brakes.
Only use DOT 4 brake fluid in your VW Passat. Do not use DOT 5 because it's not compatible!
22mm or 7/8 open wrench
T25 and T45 torx bits
16 and 19mm open wrenches
11, 12, or 13 mm flare/open wrenches for the brake line unions
paper towels
Brake fluid eats paint so make sure you blot off and wash away any spilled brake fluid.
Because you're working with metal tools right next to the positive terminal of the battery, first disconnect remove and cover any metal rings, watches, or loose necklaces before working with the battery since touching the terminal to a ground can cause injury or damage, see the TOS Agreement for the full legal disclamer. Remove the battery tie down bracket (1x 13mm bolt). Then remove the negative terminal first, then the positive terminal. You can then remove the battery. This will also give you a lot more room to work. See 1000q: battery removal for more details and pictures.
Open the driver's side door and pry off the fuse box cover at the end of the
dashboard. Remove
the 2x torx screws holding the side of the lower knee trim panel.
Remove 2 more torx screws in the front of the panel. There are a few spring clips that may pop out when you pull off the panel, make sure to save them. Once it's off
disconnect
the headlight switch, dimmer switch, and obd2 port plugs. You now have
easy access to the brake pedal.
You can also remove the headlight switch by (in the off position) pushing the
switch part forward, then turning it about 1/8 turn, then pull it straight out.
Before loosening the brake pedal, note it's original position so that the rod is the same length during installation. The overall length should be 158.5-159mm depending on what model you have. Also note the position of the pedal against the brake light switch.
Loosen the brake booster from the pedal by disconnecting the pedal rod end. Loosen the 19mm nut while counterholding the 16mm rod end. Once the nut is loosened, unscrew the rod end (the 16mm part) until the end is disconnected from the rod. You can't unscrew it completely until the brake booster is loose, just loosen it now and you should be able to get more play on the brake booster.
Here is the brake pedal/booster/cylinder removed from the car. The
long T45 bolt anchors to the pedal assembly. The pedal rod can be seen to
the left of the
brake pedal, going into the brake booster.
Open the hood and clean the area around the brake master cylinder, brake lines, and reservoir. You want to avoid getting dirt into the hydraulic system since this can damage the seals. Get as much fluid as possible out of the reservoir with a vacuum or turkey baster.
Remove the 1x T25 torx screw holding the reservoir to the master cylinder. Stuff some paper towels around the base and pull out the reservoir. Wipe up/wash out any spilled brake fluid.
Remove the vacuum line for the brake booster.
First loosen all the nuts/bolts/brake line unions before removing them. This will hold it steady while you loosen the other nuts/bolts, etc.
Then remove the 22 mm nuts holding the master cylinder to the brake booster. You can also use a 7/8 wrench. Also remove the 2x T45 bolts holding the booster to the brake pedal assembly. Once they're loose, you can tilt the booster and master cylinder to pull them out.
Clearance is better on the upper bolt but clearance is very tight on the
lower bolt. I used a T45 bit with a 1/4" 6 sided wrench to fit in
there. A 12
sided wrench may strip the bit/wrench. The bits of paper towel are used
to wedge the bit in and prevent it from falling out.
In the above assembly picture and below, you can see how the torx bolt and nut are independent
and the tight clearance on the lower bolt.
Once they're all removed you should be able to tilt the brake master cylinder and brake booster for removal.
Installation is the reverse of removal.
Make sure the brake master cylinder rod is going straight into the brake booster during installation. Lightly install everything. If the brake pedal is hitting something and getting jammed it means the rod is crooked so don't tighten it down.
Remember to reinstall the vacuum line. Check for any spilled brake fluid.
Also flush the drain line below the brake booster. If water builds up there it could leak into the interior and damage the control module under the driver's seat.
(use threadlocker if you
reuse them since they are self locking fasteners)
22mm nuts: 36 ft-lb
T45 bolts: 18 ft-lb
brake line unions at the master cylinder: 18 ft-lb
How do you get in that tight spot to loosen the brake booster reservoir on your Passat? Share your experience in VW Passat TDI forums or to report broken links or pictures.