"I don't feel good, doctor; what's wrong with me" :)
Need some more information, but here's some suggestions. Based on the age of the vehicle, there could be a ring of rust and crud buildup on the piston inside the caliper. This won't cause a major problem until you install new brake pads. To fit the new, thicker pads in, you have to push the piston back into its bore. Movement of the piston is the self-adjusting feature that takes up free play as the pads wear over time. When you push the piston back in, the ring of rust slides under the square-cut rubber seal. Under normal operation, the seal sticks to the piston and bends a little when the brakes are applied. When the brakes are released, the seal tries to straighten out and it pulls the brake pad away from the rotor just a little to prevent friction. The rust ring causes the piston to stick and not retract properly causing that brake to drag. Sometimes the brake rotor will wobble a little over bumpy roads, and that will push the piston away from the rotor to help release it.
To test a caliper or to install new pads, use a screwdriver as a lever against the rotor to pry the piston back into the caliper housing. It should move very easily with moderate hand pressure. It's easier to do this now before removing the caliper from its mount. A lot of people use a big c-clamp to crank the piston in. If you NEED to use a clamp, the caliper has rust buildup on the piston. Just replace the caliper; you should not NEED a clamp. In fact, I proved to my students that most pistons can be retracted with your hands after it's removed from the mounting.
In the past, we would take the caliper apart, clean it out, replace the seal and dust boot, (around $5.00), and clean up the piston. If there is any sign of rust, or lifting of the chrome plating on the polished sealing surface, the piston must be replaced, (about $25.00). That was a common part of a brake job in the 1980s when rebuilt calipers cost around $90.00. Today you can find quality rebuilt calipers at the parts houses for under $20.00 and they come with a warranty, so rebuilding them in the field is not cost-effective. You can also find "loaded" calipers that come with new pads already installed with anti-squeal lubricant and anti-rattle clips in the proper places.
Another cause of locking brakes is fluid contamination. Any petroleum product such as power steering fluid, engine oil, or transmission fluid will cause the rubber components to swell. Pull off the caps on the master cylinder. If the rubber seals under the caps are soft and mushy, and blow up so big you can't get the caps back on, the fluid is contaminated. In this case, all steel lines must be flushed and dried, and, ... This is critical;... ALL RUBBER PARTS MUST BE REPLACED. That means all front and rear rubber brake hoses, all calipers and wheel cylinders, the master cylinder, the combination valve on the frame rail by the master cylinder, the height-sensing proportioning valve, if you have one, near the left rear wheel or back axle, and the anti-lock brake hydraulic assembly if you have anti-lock brakes. The master cylinder and valve assemblies have rubber seals that become damaged from petroleum products. This repair can easily cost one to two thousand dollars; a very costly mistake. (Some unscrupulous sellers will add a little transmission fluid to the brake fluid to hide a defective master cylinder. Internal leakage will cause the brake pedal to slowly sink to the floor as you hold steady pressure on it. The transmission fluid causes seals will swell causing better braking, ... At first. After a few days, "you got junk")!
Fluid contamination will eventually cause more wheels to lock up. More importantly, as the seals swell in the master cylinder, they grow past a port and block it off just like if you held the brake pedal down an inch or two. Brake fluid is trapped and has just enough pressure to lightly apply the brakes. The heating of the brakes causes the fluid to expand. Since the fluid is trapped by the seals in the master cylinder, the pressure builds some more so the brakes apply harder and more heat is generated. It's a vicious circle, but the good news in your case is you are only having the problem with one brake, not all four. For this reason, I'm hoping fluid contamination is not your problem.
By the way, brake fluid can be contaminated by filling the fluid with a funnel that was wiped out after using it to fill engine oil. The residue in the funnel is enough to cause problems. Plus, the placement of some master cylinders necessitates the use of a funnel to fill the fluid.
If the fluid appears to be NOT contaminated, but you can't turn the brake rotor or wheel by hand, (but the other three wheels turn freely, loosen the bleeder screw on the locked up wheel. If you see a tiny spurt of fluid then the wheel turns easily, the piston just released. It had trapped fluid holding the brake on. Another related symptom is you can drive away at first, and everything is fine until the first time you apply the brakes, then the lockup condition starts. Usually the brake will grab harder and harder as you keep driving. But, if yours seems to release a little as you drive, or it released when you opened the bleeder screw, suspect a collapsed brake hose at that wheel. We read about this all the time, but in my 25 years as a technician, I never, ever actually saw this until two years ago. Then I saw two in two weeks! One was on my '89 Voyager that had been parked all winter and one was on a student's eight-year-old Neon. Mine took a long time to diagnose. The problem was growing, (expanding) rust buildup inside the metal bracket attached to the center of the rubber brake hose. By opening up the crimp a little with a channel lock pliers, the hose opened up and worked fine. Under foot pressure, you can easily build up well over 1000 pounds of fluid pressure which forces its way through the hose restriction so the brake applies. The square-cut seal in the caliper isn't nearly strong enough to push the fluid back so in effect, you have a built-in check valve! If opening the bleeder screw releases the brake, try opening up the metal bracket in the middle of the hose.
If you find any signs of cracking or fraying on the outer casing of the rubber hose, just replace it now. Don't wait for it to pop when you need it most!
One last comment. If you recently replaced the pads and noticed one was worn down to the metal backing plate and the other one had a lot of lining material left, the caliper is not sliding back and forth properly. A bent or cross-threaded mounting bolt can cause this, as well as a lack of proper lubrication. This is an area that definitely separates professional technicians from do-it-yourselfers. Many people can take things apart and figure out how to install new parts, but professionals use lubricants placed on the caliper bolts, the caliper slides on the spindle, and between the pads and caliper housing and piston. The pad lubricant reduces the tendency for higher-quality linings to squeal, and the caliper lubricant allows the caliper to slide freely. This is especially important on Ford trucks that come with built-in sticking brake calipers. The piston only pushes the inner pad into the rotor. The reaction force pulls the caliper inward to apply the outer pad. If the caliper can't slide freely, due to rust and dirt buildup, (or in the case of older Ford trucks), the outer pad will always be in contact with the rotor causing it to wear out much faster than the inner pad.
Dirt and hardened grease on caliper mounting bolts can be cleaned up on a wire wheel or with brake cleaning solvents. Rust pits that have lifted the chrome plating on the bolts can not be cleaned and the bolt must be replaced to insure the caliper can slide freely.
Be sure to wash off any grease or fingerprints from the brake rotor and pads to prevent brake squeal.
Thursday, March 19th, 2009 AT 4:19 PM