Calipers caused a lot of problems in the '70s, but they're pretty reliable now. The biggest problem is a stuck piston. The symptoms of a sticking piston include 1) the vehicle will pull toward the non-sticking caliper under light braking, and 2) it will pull toward the sticking caliper AFTER normal to hard braking. In the first instance, the sticking piston will not apply so the car pulls toward the working caliper. In the second instance, the sticking piston applies because of the high fluid pressure, but sticks and doesn't release so that one brake stays applied when you're not pressing the brake pedal.
As the brake pads wear, the pistons move out of the calipers to self-adjust and maintain a high, solid brake pedal. Sometimes dirt and moisture get behind the dust boot and build up on the exposed part of the piston. Rust can also develop on the piston causing the chrome plating to lift and make rough spots. The problem occurs when you install new pads and have to push the pistons back into the calipers. Doing that moves the rust and dirt buildup under and past the "square-cut seal". This is when the sticking problem starts.
When you replace the pads, start by using a flat-blade screwdriver as a pry bar to push the pistons in before you unbolt and remove the calipers. This will move the brake fluid from the calipers back up to the master cylinder. Professionals will never fill the master cylinder during routine oil changes / maintenance to leave room for this fluid.
Some people will tell you to use a C-clamp to push the pistons back into the calipers. That should never be necessary. If you find that you MUST use a C-clamp, give up; you got junk. Some people can squeeze them back in by hand, but anyone can apply more than enough pressure with just a small screwdriver. You'll have to grunt a little, but with steady pressure, the pistons should move in smoothly.
One important note. Lately there have been a lot of posts about sticking calipers, and replacement doesn't solve the problem. I ran into two of these last year, one on my Caravan that sat all winter. If you have front rubber brake hoses that have a metal bracket supporting the hose midway along its length, rust can build up inside the crimp and constrict the hose. Heavy pressure on the brake pedal will force enough fluid through to apply the brake, but the piston won't be able to release because of the trapped fluid. The brake will release if you open the bleeder screw. You can fix this very easily by using a channel lock pliers to open the crimp just a little so it doesn't squeeze the hose shut.
Be careful to not twist the bleeder screw off. If it doesn't come loose with a 1/4" ratchet / 6 point socket, or a 6 point box wrench, hammer around it to distort the casting slightly, then try again. A more aggressive trick is to heat the bleeder screw with an acetylene torch until it's red-hot, then squirt water on just the screw to cool and shrink it. A propane torch rarely works because it doesn't get hot enough.
When replacing pads, you must also inspect the chrome-plated caliper mounting bolts. They should be replaced if there are rust spots because those will prevent the caliper from sliding sideways and applying even pressure to both pads. Typically the inner pad will wear faster when the caliper can't slide freely. These bolts should be lubricated with high-temperature brake grease. "Rusty Lube" is a common trade name, but there are many others available from the parts stores.
The next thing to inspect is the pad mounting surfaces. Grooves worn into the mounting knuckles will cause the pads to stick and they might not apply properly. These surfaces should also be lubed.
To prevent brake pads from squealing, professionals will prepare the pads and clean the caliper to pad contact points. The leading and trailing ends of the linings should be ground slightly to a 45 degree angle. Some new pads are already ground to prevent the "fingernails-on-the-blackboard" effect. If yours are not, just remove a little material with a file, sandpaper, or even by just dragging them across some concrete. All you're trying to do is remove 1/6" of the sharp edge. Experience has shown that if you can prevent the pads from squealing during the break-in period, they won't tend to squeal later either. Don't get carried away. The sharp edge acts as a squeegee to remove water and prevent brake fade after driving through deep water.
To clean the calipers, run a flat file across the pistons where they contact the pads, and across the outer fingers that contact the outer pads. You don't have to shine these surfaces up; you're just removing any high spots that could prevent the pads from fully contacting the pistons and calipers. Brake grease must also be applied to these contact points. The pads are going to vibrate, and the grease will allow them to slide easily without screeching across the pistons and caliper fingers.
Don't get any grease on the pad or rotor friction surfaces. Wash off any fingerprints too. Once that grease gets hot, it will soak into the linings and cause a squeal that you'll never solve.
Friday, May 29th, 2009 AT 1:19 AM