Maintenance Q&A
Brake testing and bypass filters

By Paul Abelson, senior technical editor

Q. The southbound Kansas City, Kan., scales have a new brake inspection machine. I don’t know if they need to pay for it or are just teaching a bunch of rookies to nitpick. They’re even writing up reflective tape and tried writing us up for signing our logs before the end of the day.

The brake machine is a pull-on type that has rollers on it. They spin your wheels one axle at a time while you apply brake pressure, lightly at first, then hard pressure to simulate a sudden stop. I’m not sure what I think about that.

I have the old manual slack adjusters (don’t want to hear any criticism about that) and would have to readjust my brakes after something like that, and don’t like the idea of them wearing down my brake shoes anyway. What do you think about this technology?

A. The machine you describe is a performance-based brake tester. The purpose of a brake inspection is to ensure that your brakes will safely and controllably stop your truck. The traditional visual inspection may identify problems or it may not. Stroke travel is just one factor involved, yet it is relied on as the primary indicator of brake condition.

A performance-based brake tester, as you described, “measures brake force as the wheels are turned by the tester and the brakes are gradually applied until peak brake force is achieved,” as described in TMC Recommended Practice RP658, target deceleration value guidelines for performance-based brake testers.

The RP also describes some of the limitations of performance-based brake testers. For instance, tests at low speed and low temperature cannot necessarily predict high-speed, high-temperature performance. They are not a substitute for visual inspections and provide no information on lining wear. They cannot predict stopping distance. They are a supplement to visual checks, not a replacement.

In addition to roller brake dynamometers, flat plate dynamometers “measure peak brake force as a vehicle is driven into it at low speed and brakes are applied,” according to the RP. You may come across some inspection stations with flat plate performance-based brake testers.

The regulations have precise braking force and deceleration requirements. The only way to determine whether the vehicle meets the requirement is by using a brake tester.

TMC developed diagnostic guidelines for performance-based brake testing. When they are followed during a test conducted or supervised by a well-trained inspector, your brakes and tires should suffer no more additional wear than when you stop at a traffic light from no more than 30 mph. Your manual slack adjusters should not need additional adjustment.

It’s interesting that you mentioned drivers being written up for reflective tape. Back in the late 1980s and early ’90s, I chaired a joint TMC-SAE (Society of Automotive Engineers) subcommittee on material specifications for retro-reflective tape and devices. Our work was incorporated into SAE J594 Reflex Reflectors and, by reference, into 393.11 and 393.13.

Adhesive-backed retro-reflective sheeting is expected to survive seven years in normal use. This includes adhesion, light reflection, fade resistance, abrasion resistance, etc. Federal Motor Vehicle Safety Standard (FMVSS) 108 requires that retro-reflective material be in place on trucks, tractors and trailers. It is the operator’s responsibility to maintain the material and replace it when it ages or is damaged.

I have had several opportunities to replace brittle, fading conspicuity tape. As with most adhesive-backed films, I find the best way to remove the old tape is to soften the adhesive with a heat gun, then scrape the tape off with a plastic putty spreader. A metal tool could damage the painted metal or aluminum, so be careful and gentle.

Q. I found a used bypass filter at a parts yard. I couldn’t say no to the price. I’ll mount it on the chassis, but I need to know how to connect it. Also, where do I find replacement filter cartridges?

A. During a conversation about your inquiry, the unit you described is either from Como’s Black Gold line (800-451-0028) or Harvard Filters (800-523-1327). The products are very similar and replacement elements are interchangeable. They provide filtration in the 1 to 5 micron range.

The elements for both units are made of special filter paper, tightly wrapped around a mandrel. When oil under pressure fills the container, it’s forced through the filter where contaminants are trapped in the paper filter medium. The clean oil is returned to the oil sump.

If you have an external hose to an oil cooler, you can clamp a T-fitting into the line. Then run a hose from the fitting to the filter.

A better way is to use a fitting that goes into an oil gallery in the engine block. Again, connect your hoses to the fitting and run the line to the filter.

I recommend you do oil analysis between oil changes if you intend to extend drain intervals. Several oil testing labs offer an engine block fitting with a T that has a spring-loaded ball valve. Pushing in a needle (used to inflate footballs and basketballs) will open the ball valve and allow oil to flow through a short length of plastic tubing into a sample jar.

Although its parts are not interchangeable, Gulf Coast Filters (800-398-8114) operate on the same principle. LL