Off Balance

This cross-section from Bendix shows the inner workings of a typical relay valve.


Sealco Air Controls
(602) 253-1007

(248) 435-1000

Bendix Commercial Vehicle Systems
(440) 329-9000

The most serious consequence of unbalanced brakes is brake failure on a long downgrade. But even if you never run across the Rockies, brake balance is nothing to ignore.

Lack of maintenance of brake system parts, replacement with the wrong part and improper spec’ing can all cause pneumatic balance problems. The result will be that one axle will apply before, and usually release after, another. Because this affects safety and maintenance costs, it can be a source of major trouble.

Brakes convert the truck’s inertia to heat. When all 10 brake drums work in unison, drums and linings remain cool because of the large amount of metal exposed to the air. When only six or eight drums do most of the work, they run hotter than they should. On a long grade, the result can be brake fade and the panic of a runaway. Even on level stretches, when an axle is doing more than its share of the work, the result is much shorter brake life because abnormal heat accelerates wear very quickly.

When you apply the brakes, the air pressure that comes through the treadle valve takes time to get to all axles. This problem is minimized with relay valves. These pneumatic devices throttle air into the brake chambers at each group of axles in direct proportion to what your foot asks for. Both the drive axles and the trailer axles of a five-axle tractor-trailer have a relay valve. On ABS-equipped trailers, the relay valve is incorporated into the ABS modulator. Relay valves are subject to wear and especially corrosion from brake system moisture.

Every relay valve has operating characteristics such as crack pressure, which is the air pressure from the treadle valve required to make it open. Even after the valve opens, it regulates the output pressure that applies the brakes in relation to the control pressure from the treadle. The relationship between treadle pressure and output pressure changes when you release the brakes, a phenomenon called hysteresis. If all these characteristics match, every brake on your rig will do its fair share of the work. If they don’t, you’ll have heat and wear troubles on some of the brakes.

All brakes should start to apply (and release) at pressures within 2 psi of one another, say Jim Szudy of Bendix Commercial Vehicle Systems and Paul Johnston of ArvinMeritor. This characteristic is especially important as application pressure rises and drops between 10 psi and 40 psi, the range where most braking is done.

Johnston says that control pressure is measured at the gladhand between tractor and trailer. “This point in the system is very important because the entire system apply-and-release characteristic is established here,” he says. Any testing is done with slow application and release of the pedal, as this is the way most braking is done. This is also most likely to show slight variations in crack pressures that exceed the 2 psi standard.

Szudy says that serious testing observes the threshold pressure – the point where the linings actually contact their respective drums and where they release. The pressures supplied to all the diaphragms should be a maximum of 10 psi apart at a given control pressure, he says.

The best time to be concerned about brake balance is when ordering a new tractor or trailer. You can avoid problems of mismatched components, says Johnston: “Specify common brake type and manufacturer on all axles.” Spec’ing a tractor that will pull trailers spec’ed by a carrier is a special case. The best thing to do, Johnston says, is to survey the existing trailers “regarding brake type and size, tire sizes and friction materials. Review this data with the tractor dealer and seek recommendations from the OEM to cover the tractor.”

If you are experiencing balance problems with an existing rig, the cause could be mismatched components that can be replaced with others that will restore balance. Chuck Eberling, senior staff engineer with Bendix, notes several points regarding how a system should be set up. There should be no pressure limiting valves on the front axle because it is “just as critical as the drive and trailer axles” for total braking. DOT regs keep them off late-model trucks. If you have these on an older vehicle, consider removing them.

The front axle should have a zero-hysteresis quick-release valve. Where separate valves are used from side to side, they must have identical characteristics, “or you could have a steering pull,” says Eberling. Johnston says to “specify a ‘0 psi’ crack pressure quick-release valve in the tractor-to-trailer control line, ideally located in the gladhand coupling or tractor protection valve.”

The most important point: A rig should have “the same control valve characteristics on all the axles,” says Eberling. This means nearly the same pressure is output to the diaphragms at each pressure over a wide range of control pressures. The tolerance is 10 psi maximum from the lowest to the highest. The valves also must have similar hysteresis characteristics. This refers to the width of the band between the output pressure as measured when you’re applying versus when you’re releasing.

Brake hardware can have a significant effect on balance, even if the pressures are OK. No amount of pressure adjustment will do much good unless the brakes are sized to match the load. A tractor-trailer normally carries 34,000 pounds on each set of tandems, even when the buyer may specify a 40,000-pound tandem drive axle for longevity. Put 34,000-pound foundation brakes on the tandem if that’s what your usage rating is.
Keeping pneumatics in balance requires extra diligence. The payoff comes with a safer operation and longer life of tires and braking components.


What kinds of problems will clue you in to pressure balance trouble?

“Watch for vehicle reaction during braking conditions – excessive trailer run-in to the tractor or tractor nose-diving can indicate imbalance,” says Paul Johnston of ArvinMeritor. Also, “Inspect brakes for signs of excessive or different wear patterns.”

Leakage in a relay valve is a strong indication of trouble, says Jim Szudy of Bendix Commercial Vehicle Systems. To check this, have someone gradually apply the service brakes with the vehicle safely chocked. Listen and feel for external leaks at the exhaust port of each relay valve during application (not release).

When replacing the valve, says Szudy, “Make sure it has the same key crack pressure and other valve characteristics. These include the curves of apply and release pressures and hysteresis.”

Here are other key preventive maintenance points:

MAINTAIN THE AIR DRYER. “Maintain clean systems by using air dryers or other means to control water and contamination,” Johnston says. Dryer maintenance mostly means replacing the desiccant cartridge as needed.

WATCH FOR LEAKS. Leaks will obviously upset balance because the pressure won’t ever get to the diaphragm. They also pack a double-whammy, says Chuck Eberling of Bendix: “Leaks tax the charging system. The dryer then saturates with moisture.”

MAINTAIN HOSES. “Make sure tractor-to-trailer lines (coiled tubing or hoses) are not crimped,” Johnston says. “Replace with same type and size. Using longer-than-necessary coiled tubing between vehicles can upset pressure balance as well as timing balance.”

Small-fleet owner Bob Maddox of Nick and Dee’s Trucking of Hereford, Texas, says he discovered a pressure balance problem was caused because a pressure hose replacement had a different inside diameter.

CHECK ENTIRE BRAKE SYSTEM. Balance involves all aspects of the braking system, not just air pressure. Slack adjusters, diaphragms, the mechanical condition of the camshaft and bushings and return springs, and the lining and drum type all affect braking force and therefore the balance among axles. So, when doing any brake work, make the job thorough by replacing all worn parts and using only OEM ones or parts known to be fully compatible.

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