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Old 04-05-2016, 03:35 AM   #9
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Join Date: Jan 2006
Location: NYC
Posts: 41,057

2003 L-Series 3.0L Sedan
Default Re: 97 SC2 (No ABS) Brake Pedal goes to the floor

Front left caliper;

Front right caliper;

While it was nice of you (or anyone) to video capture applying brakes with and without the engine running (without wheels mounted and rotors locked in place with lug nuts), the video may be giving false impressions.

The main caliper assembly is designed to "float" over two caliper pins mounted with plenty of clearance inside two rubber boots holding high temperature grease. The caliper is allowed to freely align itself over these two pins while straddling the rotor disc. The disc is relied on being bolted in place to the wheel hub to have less than two thousandths of an inch (0.002") lateral runout (disc warp) with the wheel bearing not having any slop. The disc is presumed in most cases to run true. The caliper is allowed to float sideways to automatically align itself to the rotor disc. As soon as braking is applied, the piston on one side of the caliper is pushed out against that side of the brake pad while the caliper moves sideways until the other brake pad contacts the other side of the rotor disc. At this moment the caliper is aligned and acts as a fat 'C' clamp to compress brake pads against the rotor. The self aligning caliper depends on the greased pins to provide self alignment at all times. In reality, once brakes are serviced with new pads or rotors, the piston is retracted with a lot of space between brake pads and rotor disc. This large air gap is gone as soon as brake pedal is applied; brake hydraulics forces brake fluid into the caliper to push the piston out. As soon as the first brake pedal is applied, all the slop is removed with the caliper aligning itself to distribute pressure to both brake pads. When braking is released, the caliper piston and very imperceptible rotor runout forces the piston back into its caliper housing - just a few thousandths of an inch and not visibly noticeable but enough to see the brake pads still slightly rubbing against the rotor faces. This is acceptable and not considered drag or pad/rotor overheating. The caliper is effectively aligned for the life of these parts and continually moves very slightly as brake pads and rotor wear down. When new parts are used and the caliper piston is fully retracted into home position, you can move the caliper sideways and twist it in place to see how much slop the caliper pins allows for freedom of movement. As long as the pins and grease allows free movement, the caliper will self align automatically to allow pads to wear down evenly.The video is showing a false impression as if the rotor is moving sideways against the fixed caliper. Are you sure the rotor is bolted and not free to move sideways?

Do not assume what you see is correct as its shown with the caliper not moving and appears as if the rotor is being moved sideways. Unless you mount the wheel back in place or ensure the lug nuts are bolted to hold the rotor in place against sideways movement, you're being distracted with misrepresentation of a nice video. Remember this, the rotor disc is fixed against sideways movement when a wheel is bolted on. The caliper is always floating across the two "slide" pins to allow it to self align. Try imagining a fat "C" clamp with two holes for pins, one at the top and the other on the bottom. The holes for the pins have a rubber boot to hold grease and large enough to have a sloppy fit so the "C" clamp can move axially and perpendicular - all to allow as much freedom to move sideways on these two pins. The pins are bolted to the (separate) caliper frame. When brakes are applied the caliper slides slightly, the piston moves out and presses against the brake pad that presses against the rotor that presses the opposite pad resting against the caliper body. This large "C" clamp simply floats into position to clamp both brake pads against the rotor disc to slow it down from turning.

Last edited by fdryer; 04-05-2016 at 03:46 AM..


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