- Jul 6, 2006
Autoblog Maintenance 201: Brake pad replacement, Part II
In the first half of this particular write-up, we showed the basic process of replacing the front rotors and pads on a VW Jetta. Now we'll go ahead and hit the rear brakes, since the car is already up in the air and we're already covered up to our elbows in grease and brake fluid. We'll also go through the process of flushing out the old fluid and bleeding the brakes, since this car was far overdue for that task.
As we stated in our previous post, this task isn't mechanically difficult, but it does require attention to detail and an eye towards safety. If in doubt, seek the services of a qualified professional.
The rear brakes are similar to the front in terms of outward appearance, but with smaller components (the rear brakes simply don't play a huge role in high-effort stops). In this case, the slider hardware is also somewhat different. Due to the parking brake, there's also something unique about how we deal with pushing back the caliper pistons, but that's not yet visible.
As we suggested before, it's a good idea to make an attempt at opening the bleeder valve before any other work is done. Hopefully, it cracks open easily; if not, a contingency plan will need to be developed to either extract the bleeder, or to obtain a new or rebuilt caliper.
Instead of stationary slider pins that screw directly into the bracket, here we have pins that float in the spindle assembly. The caliper is then fixed to these pins via a bolt. In order to remove the bolts, the pin must be kept from rotating via its hex. This requires the use of a particularly thin open-end wrench.
As I was scrounging through my toolbox to find a spare 15mm open-end to sacrifice for this job (such a wrench could also be purchased, but not in a small town on a Saturday afternoon), the car's owner simply attacked it with a pair of slip-joint pliers and got all of the hardware removed before I even had a chance to spin up the bench grinder. It's not the technique that we recommend, and of course it's always best to use the proper tool and save some potential hassle now or the next time that the brakes are done. However, as long as the jaws of the pliers aren't allowed to slip on the surface of the pin, it's unlikely that damage will occur, and as you can see here things worked out just fine.
With two bolts on each side removed, simply slide the caliper off the spindle, and support it using wire or a nylon "zip tie". Remove the old brake pads and discard them. Remove any hardware retaining the disc, remove the disc, and set it aside for recycling.
At this point, inspect the slide pins. Gently roll back the rubber boots and verify that there is an ample supply of fresh grease. They should move back-and-forth in the caliper bracket with minimal force; if they're hanging up slightly, it probably means that they're dry and should be removed, cleaned, lubricated, and re-installed. If they don't slide at all, you can attempt to remove the old pins using locking pliers and install new hardware, but depending on one's mechanical competency it may be easier - but not cheaper - to purchase a new caliper bracket assembly (in a future Autoblog Project Garage installment, we'll demonstrate how to save a bracket that would otherwise be heading for the trash heap).
Since the rear brake calipers contain provisions for applying the mechanical parking brake, they have a self-adjust feature built into them that requires a special tool to rotate the caliper piston as it's being pushed back in its bore. This tool, shown above, can be purchased at most auto parts stores, or obtained through Auto Zone's "tool loan" system (you basically pay the full price of the tool as a deposit, and that money is returned if you return it in good condition). You can also attempt to do the rotate-and-push chore with the tips of needlenose pliers or one of the inexpensive cube-shaped "universal" tools, but such efforts usually just lead to frustration. Buy, rent, or borrow the right tool, and this part of the job will take just a few minutes for both sides.
As also done on the front brakes, we opened the bleeder valve to allow the old fluid to escape while we pushed back the piston. This is done to prevent contaminated fluid from being forced upstream into the delicate (and expensive!) ABS module.
Note that the caliper is just hanging by the parking brake cable in this shot - it shouldn't be doing that (especially not with the pushback tool attached). Make sure that the calipers are properly supported at all times to prevent any possible damage to the brake hoses or cables.
Clean the new rotors with brake cleaner and fresh rags or towels. After cleaning any loose corrosion from the hub, apply a thin coat of grease or anti-seize compound, and slide the disc into place. Hit it again with the brake cleaner and towels just to make sure that it's free of contamination.
Clean the pads, set them into place on the rotor and caliper bracket, and then slide the caliper into place. Install the bolts using anti-seize compound on the threads, and torque them to specification.
Now, before the wheels go back on, we're going to bleed the brakes. A patient helper is required for this method, and while both vacuum and pressure bleeding can be done single-handedly, the tried-and-true manual method produces excellent results with no additional investment in tools.
We're doing this for two very good reasons - first, the fluid in this car was long overdue for a change. While those in drier, more consistent climates can go many years without worrying about the fluid, humid and often-changing conditions such as those here in the Midwest allow the fluid to "pull" moisture into the system. Left long enough, this forms corrosion in the system, and loose particles will float around and cause all sorts of problems. Considering the minimal cost of bleeding one's own brakes, it's not a bad idea to consider it a yearly maintenance task. At the very least, do it every time a brake job is performed. While some vehicles allow access to the bleeders while the wheels are on the vehicle, it's certainly much easier to do the job while the wheels are off. Combine this with a tire rotation for maximum effectiveness.
Most vehicles with ABS can be bled in this manner, but you should verify with your service manual that no additional steps are required before proceeding.
Start by locating the brake master cylinder and reservoir. It will be on the left side of the firewall, and in this case, it was mounted rather low and behind some plumbing. Wipe off the cap and surrounding area with a clean rag before removing the cap, and if you set it down somewhere, make sure to place a rag underneath it.
This would be a good time to note that brake fluid is an excellent paint remover, so do not allow it to contact your car's finish. If a spill does occur, immediately clean it up with water and a clean rag, being careful not to wipe it across a wider area.
Start by removing the old fluid from the reservoir using a vacuum pump (shown here) or a turkey baster. Whatever you use, don't return it to the kitchen drawer when you're done.
When the old fluid has been removed, top off the reservoir with fresh clean fluid of the appropriate type, and secure the cap before continuing. Selecting a fluid to use can be a confusing topic; start with your manufacturer's recommendation of either DOT3 or DOT4, and if you want to get passionate about it, here's some additional reading on the topic. High-performance brake fluids are great, but simply bleeding the brakes with any fluid on a regular basis already puts you in the top 1% or so of car owners.
Starting at the corner of the car furthest from the brakes (usually either of the rear wheels), we loosen the bleeder with a box wrench, and then gently snug it back up.
Keeping the wrench on the bleeder, we then run a hose from the bleeder down to the appropriate catch canister. A quart-sized Gatorade bottle works much better than a 20oz soda bottle for this task, and the use of clear tubing makes it much easier to see bubbles.
With the bleeder closed (gently tightened), have your helper stand on the brake pedal. When they indicate that pressure has been applied, open the bleeder just enough for fluid to start flowing. Once your helper indicates that the pedal has reached the floor, have him or her hold it there while you tighten the bleeder. Shout back to the helper that they can now release the pedal.
Repeat the process until clean fluid is observed to be flowing from the bleeder, and that no bubbles are present. Then do the same at the remaining three wheels.
Keep a very close eye on the fluid level in the reservoir, as it must be kept at least partially filled at all times. If it runs dry, the bleeding process must be started all over again, so please check it on a very regular basis. Also make sure that the reservoir cap is secured before anyone touches the brake pedal, or else you may observe a glycol geyser.
If you haven't serviced your brakes recently, you may note that the fluid comes out looking more like muddy water or even motor oil. That discoloration is the result of internal corrosion, so it's extremely important to bleed the brakes and flush out the old fluid before it starts turning nasty colors.
After all four corners have been bled and the reservoir is topped off, put the vehicle in Park or Neutral and start the engine. It may be necessary to pump the pedal once or twice to bring the pads into contact with the rotor, and after that a solid pedal feel should be observed. If not, air remains somewhere in the system, so it's time to repeat the process. Do not place the car into service until the pedal feel is solid. Top off the fluid one last time if required.
Once the wheels and tires have been re-installed and the car lowered to the ground, it's time to "bed" the pads as soon as possible (like, before making a beer run). The goal is to transfer a thin and uniform layer of brake pad material to the brake rotors. The procedure should be provided by the pad manufacturer; if not, the following has worked for us:
1) Perform six moderate near-stops from 35 MPH to 5 MPH. Do these in quick succession. Try not to come to a complete stop.
2) Perform two additional hard near-stops from 45 MPH to 5 MPH or so. Some brake fade, burning smells, and even a bit of smoke may be experienced.
3) Cruise around for 5-10 minutes with minimal use of the brakes. Whatever you do, try to keep rolling at all times until the brakes have cooled. This is to prevent "imprinting" the rotors with the pad material, which can result in brake pulsation.
If the brakes have gotten hot enough to properly bed the pads, one should see a hazy grey appearance where the pads contact the rotor. There will be a bit of a blue-purple color on the rotor surface, right along the edge of the swept area.
There are some nearby backcountry roads that we use for bedding brakes, where we can perform these near-stops and roll through some uncontrolled intersections afterwards without creating a hazard. You will certainly not want to do this in a populated area. Vehicles with higher-performance brake systems may require more or harder stops to bring the system up to maximum operating temperature. For those interested, StopTech provides additional reading fun on this exciting topic.