No it is not implied. A lot of high-quality trucks have rear disc brakes. Brakes are one of my three specialty areas but I don't make it a point to memorize every brand and every model and every option out there; that's why I ask.
Since you're being so secretive about the details, I also have to ask about the procedures you followed. Did you machine the drums? Did you cut them all the way to the face or just as far as the old linings had been contacting? If you left a ridge of rust, the new linings, which are often not centered exactly the same as the old ones, will ride on that ridge and not sit squarely on the friction surface of the drum.
Was there any sign of fluid leakage? Did you pull the dust boots back to check for brake fluid leaking from the wheel cylinders? Axle grease can leak from the seal too. That will lead to brake squeals as the linings grab and release rapidly.
Did you file or sand off the sharp leading edges of the linings so they don't act like dragging fingernails down the blackboard?
Did you check that there is at least 1/8" of free play in the parking brake strut? If that strut is tight, the parking brake cable is rusted tight in the partially applied position; something Ford is famous for. The shoe frames must also be contacting the large anchor bolt at the top of the backing plate.
Did you check for grooves worn into the backing plate from the shoes sliding back and forth? There's six "lands" that the shoes slide on. When grooves wear into them, they can prevent a shoe from fully applying and from fully releasing. Anything that prevents even pressure on a lining can allow it to vibrate and set up a squeal.
Drum brakes by their very nature have a tendency to not squeal because the rotating drum is trying to release the rear shoe. A real common mistake many do-it-yourselfers make is they put the shoes on in the wrong positions. The shorter lining always goes toward the front of the truck, and the longer one goes toward the rear.
Did you check for the presence of round rust spots on the inside of the drums where they sit on the axle flange? If those were not scraped off before the drums were machined, you may have machined a warp into them, depending on how it was mounted on the lathe. You may not feel that in the brake pedal, but it will cause the shoes to move forward and backward with each wheel revolution. Professionals use high-temperature brake grease on the six lands, those surfaces the shoes slide on. That grease will wear away eventually so the shoes sliding back and forth can set up a squeal. That will often start days or weeks after the brakes were replaced. If you put them together dry, the squealing could start right away.
If you adjusted the new shoes up too tightly, the linings and drums could be glazed. That alters the friction characteristics so the linings will slide over the drums instead of bite in and grab them. Many people are mislead when they go according to feel when they adjust up the shoes. The new linings have the same outside diameter as the inside diameter of a new drum. Worn or machined drums have a larger inside diameter, so the linings only make contact on a very small patch near the center. That small amount of friction is hard to feel when turning the drum by hand so many people over-tighten the shoes. The constant contact while driving can lead to glazing in that area. As the linings wear down to match the diameter of the drum, that glazing doesn't go away. The shoes will not self-adjust up higher until they're supposed to, so if you give the shoes a few weeks to wear down to match the drums, you just remove the drums and sand the glazing off the drums and linings, and don't touch the adjustments. If you do that too soon before the linings have worn to match the drums, the problem can come right back again.
What IS implied is Ford has a characteristic built-in from the factory that promotes squealing drum brakes. If you look at any other brand of vehicle, then look at your shoes, you'll see the anchor pin is not in the center of the shoe frame. It is down much lower. That allows the upper half of the shoe to vibrate much more easily and set up a squeal. That is a well-known problem among brake specialists.
Another common, but often overlooked problem is brake fade with new linings. You may not notice that with rear drum brakes but it is also common with front disc brakes, and only pertains to just having the new linings installed. If the rotors or drums were machined, that leaves them with grooves causing high and low spots. The new linings will only make contact with about five percent of their surface until they wear down to match those grooves. That can take a hundred miles or more. Many of the better shops hang tags from the rear-view mirror warning owners to take it easy until the new linings are seated. Smart service advisers at least make that verbal warning to their customers. If you take off and drive through a lot of city driving like normal, you'll have to press a lot harder on the brake pedal to get the same stopping power. That leads to overheating the linings and glazing. The result can be severe brake fade; no stopping power but the pedal is high and firm. Typically the brakes will be fine once they are allowed to cool down for a couple of hours. The friction coefficient will come back for the front pads, but the rear shoes, which contribute a much lower percentage of stopping power, will be glazed, and that won't usually clear up on its own.
Even when someone cuts corners and doesn't bother having the drums machined, the friction surfaces will not be perfectly flat and smooth so the new linings won't make full contact until they wear down a little. The same thing happens when new drums are installed. You'll see they have a much smoother friction surface than machined drums, but the new linings are rough and still won't match perfectly right away. To reduce the possibility of causing a brake squeal, manufacturers all specify the proper surface finish they want for their drums and rotors. One way to achieve that is by using a sanding disc on the brake lathe to make a non-directional finish after machining. This pertains more to rotors, but you can get similar results on drums by taking a very light, .002" final cut.
Speaking of depth of cut, .006" is the maximum that should be cut on one pass. More than that will overheat the cutting bits and make them dull. They will gouge the metal rather than slice it off, leading to a very rough finish. At less than.002", not enough heat will be removed with each chip of metal.
If the rubber strap wasn't used around the drum when they were machined, the drum will vibrate and "sing" on the lathe. That will set up a very noticeable pattern in the finished surface that can cause a grinding sound, rumbling, or squealing. If they're singing on the lathe, that absolutely must be stopped before the cut is allowed to continue. Sometimes that requires stuffing pieces of vacuum hose in the labyrinth seal.
Some drums have a coil spring around their outer surface to dampen squeals. When parts of the drum rust away or flake off, it changes its natural frequency to where we might be able to hear the squeal.
Many shops require their mechanics to replace the hold-down hardware and return springs on brake jobs on older or high-mileage vehicles. From being hot so many times, those springs lose their effectiveness and let the shoes vibrate more than normal.
A real big cause of squealing brakes is grease on the linings. That includes axle lubricant, wheel bearing grease, engine oil on your hands, and in severe cases, even fingerprint grease. One shop owner in our area makes his mechanics throw the new linings away and install a new set if he sees any grease on them. All you really have to do is wash and scrub the grease off with brake parts cleaner before the linings get hot. Once they get hot, the grease will soak in and never totally come out. Keep in mind too that drums and rotors are cast iron which is porous. Grease can also soak into them. Machining usually removes enough material to remove all of the grease from before the brake job, but grease on the new linings can soak into the drums after machining. Grease contamination is probably the number one cause of drum brake squeal.
When the wheels were reinstalled, were the lug nuts tightened with an impact gun or with a torque wrench? In my shop, a mechanic got one verbal warning if they were caught using an impact. The second time they were out the door. It's that important. Drums are more forgiving than rotors when it comes to warping from uneven clamping forces, but if a small piece of scale or other debris gets stuck between the drum and axle flange, or if those rust spots weren't scraped off, the drum will bend when the lug nuts are tightened. Over-tightening will make that worse.
Was any type of grease used on the studs? We spent a lot of time in my Brakes class on just such practices. Anyone caught on the job using anti-seize compound on lug nut studs was immediately fired. Use of that material in that application was certain to lead to a lawsuit. In my class, the tool room attendant watched very closely what the student was using that for. If it was on wheel studs, my supervisor and I both got involved in disciplinary action. Anti-seize WILL let the nuts work loose.
Some people, including myself, like to use a light coating of axle grease on the studs, especially when your daily driver is a rusty trusty '88 Grand Caravan in the heart of road salt country. But the key work is "light". More is not better. Also, many import and smaller cars use anodized studs. That electroplating is a lubricant, and some greases will eat that away. Anodized studs must not have any type of grease applied.
When you do use a little grease, it is important to turn the lug nuts on as far as possible by hand, not with an impact. When the impact is used, the grease will build up ahead of the nut, then the centrifugal force will spin the grease outward onto the friction surface of the nut or wheel. That rounded friction surface is what holds the nut from coming loose. Grease on the threads is okay but not on the friction surface. If you see any hint of grease splattered on the wheel, you know too much was used and the nuts were spun on with an impact.
By now it should be apparent there's a lot more to a professional brake job than most do-it-yourselfers realize. That's why we read about so many problems after people replace their own brakes. My list for disc brakes is twice as long.
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Monday, August 15th, 2011 AT 10:29 PM
