Brake basics
Why is it that with today’s superior machining and metals, precision controls and devices, brakes are still the primary cause of out-of-service orders during roadside inspections?

By Paul Abelson, senior technical editor

It’s simple really. The wear and tear on brakes is extreme, and maintenance is often inadequate.

Brakes must completely stop an 80,000 pound vehicle traveling at 75 mph or more in less than the length of two football fields, or traveling at 60 mph in less than one.

A lot goes on when you hit the brakes. Simple routine maintenance of your braking system can extend the life and prevent catastrophic problems on down the road.

In drum brake systems, this involves rotating camshafts to force brake shoes against drums that rotate with the wheels. The shoes, lined with high friction, temperature-resistant materials, rub against the rotating drums where friction creates heat. The drums heat up, then cool down when heat transfers to the surrounding air.

Consider the forces involved. The heating and cooling expands and contracts metals, often at differing rates. Metal parts rotate and rub together, causing wear. And the greatest wear is to linings that are designed to sacrifice themselves in the process. Slack needs to be adjusted to compensate for the lining wear.

All this happens in ever-changing conditions: Temperatures fluctuate; there’s road shock and vibration. Considering the stresses, it’s a wonder that brakes operate as well as they do.

Spending some time closely inspecting your braking system can help you find problems early – when they are easier, and hopefully cheaper, to fix.

Brake shoes should have each component inspected closely. Check the supporting structure, the brake table that supports the lining. Be sure it is free of rust and that holes are not elongated. Check flatness and perpendicularity of surfaces. Check for cracks. Check anchor and roller pin seats for excess wear.

If any of the parts show wear, replace the entire shoe assembly, along with the one from the other axle end. It’s OK to use a remanufactured set from a reputable supplier, but beware of counterfeits. There are many knock-off brake products from questionable off-shore suppliers.

If the shoe assembly checks out, you can opt to have a brake shop reline the brakes.

Brake drums have changed over the years. In order to improve fuel economy and lighten trucks, there is generally less mass in today’s drums. When scored, we used to turn them on a lathe to restore dimensions. Now that may result in oversize drums without the mass to absorb enough heat. Check allowable specs for each manufacturer’s drums.

Minor heat checking – where the surface of the drum shows fine, hairline cracks – is acceptable. But if there are deep cracks, replace the drum immediately, especially if the cracks are all the way across. If there is even one crack on the mounting flange, replace the drum.

Blue drums are caused by excessive heat from dragging brakes, swollen linings or brake imbalance. The underlying cause should be corrected. Return springs may be binding, weak or broken.

If drums appear polished to a mirror-like finish, you may have linings with the wrong friction rating. If the drums were recently turned, the resurfacer may have used the wrong tool feeds and speeds. Continued light contact or fine contamination can also polish drums. First, check everything that may affect the proper return of the linings: return springs, camshaft bushings and even the air system. If you’re going to remove the gloss yourself, use 80-grit emery cloth. It will dull and roughen the finish without removing any appreciable metal, unlike sandpaper.

Drums may have been worn out-of-round, dropped, stored improperly or improperly mounted during installation. An out-of-round condition can also occur if the parking brakes are applied when the drum is extremely hot or if a drum was improperly chucked when it was turned.

If the drum can be turned and remain within tolerances, have that done. Otherwise, replace the drum. If a drum becomes oversize through normal wear, replace it.

Martensite spots are raised, brittle black spots on the braking surface. They are caused by localized heating to about 1,400 degrees followed by sudden cooling. Brake drag and torque imbalance are primary causes. Replace the drum whenever martensite appears and have the underlying causes fixed.

Scored drums can be caused by foreign abrasion material between linings and drums, or by loose rivets. If the source is external, adding dust shields may help. Depending on the depth of the scoring you may be able to have the drums turned; otherwise, they will need to be replaced.

Grease and oil stains on drums can be cleaned, but linings should be replaced after the source is found and fixed. LL