Bottom Line
Maintenance Q&A
Balancing brake wear

By Paul Abelson
Senior technical editor


Q:I run a 2005 Peterbilt heavy-haul daycab carrying petroleum and liquid asphalt in and around New York City. I run heavy (up to 107,000 pounds). I did a full brake job when I first bought my truck back in December 2007 – new slacks, drums, shoes, everything.

I noticed immediately that, when hooked to an empty trailer without ABS, the trailer wheels would lock up easily. A trailer with ABS would still brake hard, but only the lift axle would lock up sometimes.

Recently I had to put steer axle brakes on my truck after about 70,000 miles, when I would normally expect to do a full brake job doing what I do. The drive axle brakes were all still about 50 to 60 percent.

I have gone through several sets of trailer brakes and a set of steer axle brakes but not the drives yet. No one can figure out what’s wrong. I have taken it to several mechanics – and outside of replacing random brake and relay valves and the slack adjusters a second time, nothing has helped. I have noticed that the slacks do need a periodic adjustment, but that’s three sets of slack adjusters in just over 18 months. They can’t all be bad.

I’m thinking it’s an air pressure issue, but outside of replacing every hose and valve (not to mention the time and money) where can I look?

A: First, you need to define the extent of the problem. Do a leak-down test to see if there are leaks. Then use a laser-aimed infrared pyrometer to measure brake temperatures. With a full load, brake hard from a moderate speed. Put the pedal to the floorboards. You may have to do this several times to get the brakes very hot.

Brake temperatures should be very close to each other if everything is operating properly. You tested and said your trailer brakes were within 20 degrees of 550 degrees, while tractor brakes were 270 degrees plus or minus 20. Clearly, trailer brakes are doing a disproportionate share of the braking. If all was working properly, steer axle brakes should have been the hottest, and there should not be a 280-degree difference.

Switching trailers, one would expect greater wear on the tractor brakes. Your heavy loads may bias this somewhat but do not explain that much heat differential.

I wonder about aftermarket linings, slack adjusters and other brake parts you replaced. Brake parts are among the most frequently sold counterfeit parts. Counterfeits are designed to look like original equipment. Even the packaging looks the same, but tolerances are not kept. Parts are not properly hardened and tempered and are usually made from inferior materials.

Your 2005 Peterbilt came with automatic slack adjusters. Most have a one-year warranty. Well maintained, they should last hundreds of thousands of miles. You replaced three sets in 18 months, leading me to believe they were substandard. Even using the wrong grease can seriously affect brake system components, as Bendix found several years ago when it received warranty claims for products marked with their brand but not made by them.

Mismatched brake linings can affect brake balance. Make sure the linings have the proper friction characteristics for your application and they are all from the same OEM-quality supplier.

You mentioned “going to several mechanics.” Find the one brake specialist in your area. Work with him exclusively so one person “takes ownership” of your system. Make sure to use OEM components even if they cost a bit more.

Thanks to TMC stalwarts Tom Tahaney and Carl Tapp for their ideas on your brake problem. LL



Paul Abelson can be reached at