Brake Pads (Front)
Page 2 of 2: Reinstallation
Continued from Brake Pads (Front), Page 1 of 2: Removal or, if you have just replaced your rotors, from Rotors (Front).
Step 6: Compress Piston
If your old pads were very worn (and of course they are or you wouldn't be replacing them) you will need to compress the piston in order to have the new pads clear the rotor when the caliper is reinstalled. You can buy a special tool from any auto part store to do this but I found it very easy to perform this step using an ordinary wood clamp.
In order to avoid a potential mess, open up the brake fluid reservoir and use a turkey baster to remove 6-8 ounces of brake fluid (see Figure 8). Put the fluid in a clean cup as you will need to replace it later. Leave the cap of the reservoir off.
Fig 8: Remove some brake fluid with a turkey baster to avoid spillage when you compress the caliper piston.
At this point it is helpful to locate something that the caliper assembly can sit on while you work with it. Matt and I used an overturned tar bucket for this purpose. Remove your make-shift caliper holder (coat hanger) and place the assembly on the bucket, again ensuring that the brake hose is not stressed in any way.
If you've already removed the old inner pad, temporarily return it to the caliper. Carefully center the wood clamp on the old pad (Figure 9). Slowly tighten the clamp, easing the piston back into the caliper. You should be able to do this with little resistance. If you feel too much resistance, STOP, and recenter the clamp. Continue to compress the piston until only a few millimeters of the silver piston can be seen protruding from the caliper.
Fig 9: Use a wood clamp against the old pad to compress the piston back into the caliper.
Note: If you just can't get the piston to compress, here's what you need to do. Open the rubber bleeder valve dust cap. While compressing the piston slowly open the bleeder valve. Have a receptical handy as you will lose fluid from here. If you are careful to keep pressure on the pad while the valve is open no air will enter the system. If you are not careful and air gets drawn into the bleeder valve you will need to bleed the system. I did not have to open the bleeder valve, nor did I bleed my car's brake system, so if you screw this up you're on your own.
Step 7: Clean Piston (And Rotor, If Not Replacing)
Using brake cleaner carefully spray the inside of the piston to remove any loose brake dust. Be mindful not to get brake cleaner on the rubber piston seal as it will degrade this part. Ensure that the slot that runs around the inner circumfrence of the piston (see Figure 10) is free from debris as this is where the pad clip seats.
If you are not replacing your discs, use the brake cleaner to remove loose brake dust from these parts as well. If you are replacing your discs simply wipe the new parts down to ensure that there is no residual grease or oil.
Fig 10: Use brake cleaner to remove any loose brake dust from the inside of the piston.
Step 8: Apply Anti-Squeal
The cause of brake "squeal" is counter-intuitive. Brake squeal is not caused explicitly by the contact of the pad and the rotor. Rather, brake squeal is caused by micro-vibrations in the pad as a result of extremely high-frequency "catching and slipping" of the pad against the rotor. Application of brake anti-squeal paste or spray is similarly counter-intuitive. I'm not exactly certain how these magical products prevent these vibrations, but they do. The trick is to apply the anti-squeal product to the back of the new pads (the clip side, not the pad material side). I've seen many people recommend using BMW Plastilube anti-squeal paste (BMW part number: 81 22 9 407 103) but neither of the two dealers in my area carried it. I bought a canister of aerosol product from AutoZone and found it very easy to apply and effective, to boot.
Lay your new pads out on the ground (see Figure 11) and apply a couple of coats of aerosol product to the back of the new pads. The product I used had a faint blue haze to it and after 10 minutes or so dried to a tacky finish.
Fig 11: Apply brake anti-squeal spray to the back of the new pads.
Step 9: Install New Pads
The new pads are now ready to be reinstalled. There is no such thing as a "left" and "right" pad (unlike for the rotors). That said, there is an "inner" and an "outer" pad. The inner pad has a clip on it, the outer pad does not. Your box of new front pads will contain two of each. Clip the new inner pad to the piston - ensure that you have the orientation correct (see Figure 12).
Fig 12: The new inner pad clips easily in place.
Step 10: Replace Caliper Assembly, Wear Sensor & Anti-Rattle Clip
Unlike the inner pad, the outer pad does not attach to the caliper in any way. Simply hold the outer pad in place with your hand as you slide the pad / caliper assembly back over the brake disc. See Figure 13.
Fig 13: Holding the outer pad in place, slip the pad / caliper assembly back into place over the rotor.
Replace and tighten the two caliper guide bolts (30 Nm, 22 ft-lb).
Reinstall the brake pad wear sensor being careful to properly route the cable as shown in Figure 14. The cable goes behind the strut and then loops back around, through the closed rubber bleeder valve dust cap and back to the pad. The harness side of the cable snaps in place. Snap the cover of the harness box shut. The ceramic end of the wear sensor presses easily into the new pad.
Fig 14: Properly routed brake pad wear sensor cable.
Replace the anti-rattle clip. Ensure that the clip "feet" sit on the "outside" (the side closer to the hub) of the caliper carrier. See Figure 15.
Fig 15: Proper installation of brake anti-rattle clip.
Step 11: Replace Wheel (Don't Torque!)
Replace the wheel. Hand tighten the five wheel lugs.
Fig 16: Replace the wheel and hand-tighten the lugs, only.
Lower the car back onto the ground following the reverse procedure you used to raise it (see Proper Jacking Procedure). With the car back on the ground torque the wheel lugs (100 +/- 10 Nm, 74 +- 7 ft-lb).
Step 12: Replace Brake Fluid & Pump The Brakes
Replace the brake fluid you previously removed and close the reservoir tightly. Shut the hood. Pump the brake pedal several times to restore pressure to the brake system as the new pads seat.
Congratulations! You're done!