The following condition can be present in both non-rust belt states and rust belt states. It may be found noticeably earlier in rust belt areas but can occur in any driving environment where moisture is present.
The problem is the knuckle area on the rear of these trucks has a U-shaped shim that covers the knuckle where the disc brake pads make contact. The design of these shims is highly resistive to rust, corrosion and the build up of scale. These shims fit tightly onto the knuckle and may actually require a tap to seat them in place. The problem is that rust and scale will build up between the knuckle and the shim. Obviously, the rust and scale build up will occur faster if salt is also present. A slight lifting of the shim, from the knuckle, may occur because of the amount of rust and scale that forms between the shim and knuckle.
This shim lifting issue can cause a restrictive movement problem of the disc brake pad(s).
The pads must slide freely, or they will not engage as designed. They may also hang up or stay partially engaged after application if rust has lifted the shim. Recently a two-year-old F-150 was inspected. The amount of rust under the rear disc brake pad shim lifted the shim upward 0.025 of an inch. This may not seem like much movement, but since the pads lightly contact the shim, any lift of the shim may create a binding condition with a resulting uneven inner/outer pad wear problem.
An immediate question would be “why is rust present in this area?” It is highly unlikely that the shim moves on the knuckle, as it is a snug fit. The top of the shim where the pads sit may have some pitting but really does not have any major corrosion problem. The anti-corrosive properties of the shim are still present. So what causes the rust and scale? The only logical reason is that the rust and scale build-up originates from the knuckle being raw metal. Note that when this build-up occurs it can create some pitting and corrosion on the inside of the shim.
The real solution is to coat the knuckle with a sliding caliper type of lubricant before the shim is installed. This simple procedure prevents any noticeable buildup under the shim and prevents the restricted pad movement problem described above. Some industry repair procedures now say that it is not necessary to lubricate metal-to-metal pad to shim/knuckle contact points. While this may be true on the drawing board or in a perfect world, using the correct lubricant between the ears of the metal backing of disc brake pads and shims is not harmful and may prevent problems – as long as you don’t use an excessive amount of lubricant on the metal to metal contact points.
Courtesy of The Bendix Brake Answerman.