Saturday, January 14, 2012

How To Replace Front Brake Pads

This is an illustrated instruction on how to replace front brake pads. In the four wheeled vehicles, front wheel brake pads worn out faster than the rear wheel brake linings. Because when braking, the front wheels absorb more kinetic energy to reduce vehicle speed. Also because the front wheels are using disc brakes which have smaller pads than the drum brake.

Disc brake is faster dry when wet if compared to drum brake. Water layer on disc will be thrown out by centrifugal force as the wheel rotates, so the faster the vehicle the sooner brakes dry up and normally functioning. The disc can be designed with fan blades inside to pump cooling air, so disc brake is cooler than drum brake.

The figure below shows a disc brake right after washed with water before it’s pads is replaced.


On drum brakes, because the shape of the drum, the water tends to accumulate on the inside of drum and makes it difficult to dry. To dry wet drum brake, eg. after flooded street, apply half brake or pull parking brake half while running the vehicle, the water will evaporate by the heat of friction between drum and brake lining. Drum brakes have bigger pads or shoe and better grip even with smaller brake pressure, so the piston on the drum brake is much smaller than the piston of disc brake. Brake shoe or lining of drum brake is much more durable than pads of disc brake.

Because of the above considerations it is currently designed vehicles with rear drum brakes, drum brakes work well even with low force making it suitable as a parking brake. Drum brake are generally cost less than disc brake, therefore vehicle prices will be more affordable. While disc brakes are placed in the front wheels to ensure the grip when wet.

Front brake pads are replaced when its has thickness less than 3 mm or 1/8 inch. On some vehicles, brake pads have a plate that if touching and rubbing against the disc, it will be making a sound while the vehicle is running. The sound is used as a warning to immediately replace the brake lining.

For vehicles photographed here, replacement of brake pads is performed on 70,150 km, or about 44,000 miles, pads have not been replaced since the car was new. Brake pads usage is operating condition dependent.

The thickness of the brake lining can also be peeked from crevices of wheel spokes, as shown below the worn out brake lining is pointed by yellow arrow.


The figure below shows worn out brake pads on the left, and new brake pads on the right. The new brake pad thickness is about 10 mm or 3/8 inches. A plate is mounted on two pads (in the photo at the top) is the indicator plate that will sound if it is touching and rubbing against the brake disc as a sign that it needs to be replaced. Pad with indicator plate is positioned infront of brake piston and directly pressed by brake piston.


Sometimes brake pad worn out excessively on one side only, it may happen if dirt jams caliper mechanism. So one pad is always pressed and rubbed against disc.

Below is the procedure to replace the front wheel brake pads which is applied to Honda City i-DSI. But with a little modification, this procedure can be applied to other vehicles also.

Park the vehicle, chock the wheels to help the parking brake. Jack the front wheel which needs brake pads replacement. Ensure safety, jackstand should be used to help supporting vehicle incase jack collapsed. Removed wheel can be used as a jackstand by placing under vehicle body to support vehicle if jack collapsed, image below is showing a wooden block is placed above wheel and under car body to to help jack.


After the wheel removed, check if there is leakage of brake fluid, check if any wet component by brake fluid. If any leakage from piston seal, piston housing will be wet. As brake fluid go down, the lower part of piston housing will be wet also. Leakage can also occur on nipple, and brake line connection, check for any wet part on those components.

Break fluid leakage also can be happened to master cylinder or pedal cylinder.

Clean the wheel well and all components including suspension and steering with water in order to make the job cleaner and easier.

Top image shows disc brakes have just cleaned with water, some parts still look wet. Disc brakes work by pressing or clamping pads on either side of the disc. Pads are always touching disc surface, when braking the piston will be pressing pads and clamping the left side and right side of the disc. So on the disc brake, pads are very slightly moved, only pressure is increased while braking.

While on drum brake, brake shoes do not always touch the drum. When braking, piston moves brake shoes outward and to touch the inside of drum, then the piston pressure on brake shoes will determine the braking force. If the shoes are always touching surface inside the drum, then the brake will get hot and jammed. That is why drum brake needs adjustment.

After the wheel is removed, via small window or hole on brake caliper can be seen a thin brake pad (yellow arrow) as the photo below, pad thickness can be measured through this hole to ensure replacement.


By using a screwdriver or prybar or lever to move caliper and retract piston to make pads no longer touch disc. Lever is inserted through caliper hole and into disc fan, and then pull lever to left or right (depending on the position of the wheel) to move the caliper and retract piston to release pads. In the picture below the yellow arrow indicates the position where the lever is inserted.


No need to open the bleeding valve but make sure not to spill brake fluid from the reservoir, because brake fluid will return to the reservoir. The figure below shows the brake reservoir on the left, while on the right is the clutch reservoir. If any spilled brake fluid, flush with water immediately to avoid it damages the paint.


After the caliper is moved, the lever can be inserted between the gap between disc and pad to make sure piston is fully retracted. The figure below shows using jack rod as a lever. Piston should fully retracted to make room for new thicker pads. Be careful not to damage pistorn rubber booth when using lever. Always make sure no brake fluid spilled from the reservoir.


Yellow arrow in photo below shows two caliper bolts, one upper and one lower.


Undo those two bolts that hold the brake caliper. Use the 12 mm ring spanner to loosen the bolt, and 19 mm open end spanner to hold the nut. The following figure shows caliper bolt when loosening, note how to use ring spanner and open end spanner.


Once the caliper removed, use screwdriver to remove pads. The figure below shows screwdriver is prying off the outside pad.


Below photo shows the caliper is removed, both brake pads have also been removed. Seen here two caliper bolts are put back in order not to lose, pointed by two yellow arrows. While the blue arrow point to the bleeding valve that is not opened. Make sure there are no signs of brake fluid leaking, check if any wet component by brake fluid. Usually leakage occurs on piston seal,  nipple, and brake line connection. 

Clean all brake components with water. Check all rubber boots, ensure no torn booth. Torn booth may cause dirt to accumulate and lead to brakes jam and higher fuel consumption, at high speed disc can be really hot to glow and smoke. Dirt can also cause the brakes to be difficult to operate or no brake because the piston can not grip the disc. If the dirt enters piston cylinder, it can damage piston seal causing leak and no brake pressure or hydraulic pressure. Check brake hose and make sure it is not twisted, ensure no brake fluid seepage, brakes will not working if the hose is leaking as no hydraulic pressure. Do not let the caliper hanging with the hose, if necessary tie caliper with a rope to suspension.


Check and clean anti-squeal shims, these shims are needed to prevent squeaking noise when braking. These plates are often forgotten during instalation. In the picture below there are 3 shims, on the very left is the one mounted on the outer brake pad. Two on the right are both intalled to inner brake pad or piston side brake pad, the piston pressure mark is still visible on the far right plate.


 Install new brake pads, see on below picture outer pad is already installed with its anti-squeal shim, as well as inner pad also already installed. For inner pad or piston side pad, sometimes it is difficult to install as its position is difficult to see. So fit inner pad first, because the retaining clip will furl at the opposite side, so it will be difficult to fit inner pad if the outer pad fitted first.


Below picture shows position of indicator plate (yellow arrow) of the piston side or inner side brake pad.


Reinstall caliper, fit and tighten two caliper bolts, do it with the reverse order of disassembly sequence. Always replace brake pad to the left and the right wheel at once to ensure balanced braking and vehicle will not pulled to the left nor right when braking. When tightening wheel nuts or wheel bolts, must always in criss-cross sequence. Test brake performance by running vehicle and brake suddenly, to ensure vehicle can be used safely.

Air bleeding procedure of rear brake hydraulic system is discussed in article: Brake Bleeding Practical Procedure By Only One Mechanic.

For bleeding front brake hydraulic system please read article: Front Brake Bleeding By One Mechanic Only.

If there are air bubbles in the hydraulic system, it will feel like stepping on a foam or balloon when brake pedal is stepped, or mushy pedal, and pedal travel is much longer than normal. Normal pedal travel is between 1-2 cm or 1/2 - 1 inches. As the air bleeding valve is never opened on the above procedure, and no replacement of seals, or other repairement for the hydraulic system, then there is no way for air to get into the hydraulic system. So the vehicle is safe enough to drive.

Considering that the brake fluid also serves to clean the hydraulic system, brake fluid absorbs moisture and water, the possibility of air that seeped into the hydraulic system, or the presence of brake fluid vapor in the system, therefore brake fluid must be drained periodically. Usually brake fluid is drained every 2 years, please refer to your vehicle service manual.




2 comments:

  1. Hi experts, thanks for your article "Replace Front Brake Pads", it;s useful for me.
    Could you tell me "how to clean up disc brake? thanks
    Regard's
    eka-swastya

    ReplyDelete
    Replies

    1. Hello Eka..
      Disc brake is designed to expel most dirt and debris by centrifugal force as wheel turning. But sometimes happens that contamination gets caught on the disc and caliper. Easy way to clean it is by removing the wheel and spry disc and caliper with water.

      For drum brake, you need to remove the wheel and the drum to clean it. But some drum is designed with integrated wheel bearing, so it is not very easy to remove this type of drum. Luckily, it is harder for dirt from the outside to enter, as the drum covers brake mechanism. By using pressure washer, you can clean inside drum brake by spraying water from small gap or opening without removing drum.
      Rgs, Heru

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