Today's Trucking
products Other Trucks


Posted: August 1, 2014

by Tom Howe

Let one wheel or one axle grab ahead of the others and you can slide into catastrophe. How do you deal with this in a practical manner?

Slippery road conditions magnify braking inequities that you would not notice on a dry road. Brake imbalance is a tricky proposition, especially when you remember that there are dozens of different factors that can cause it. For our purposes, we will attempt to gain a greater understanding of the brake’s plumbing system.

Brake plumbing consists of the entire air system, from the compressor to the chambers at each wheel — and everything in between. Simply enough, the plumbing is any and every component that distributes and regulates the air supply to the brakes.

Synchronized Plumbing is Critical

Studying your truck from nose to tail, you will discover that an inequity at any one point becomes critical in that everything from that point on is out of synchronization with the remainder of the system. A restriction [or enlargement!] in the plumbing directly affects the reaction time and intensity of all brakes beyond that point.

You’ll be keenly aware of this when pulling a trailer manufactured in conformity to the American FMVS 121 [after model year 1974] standard. The service line is likely to be 1/2″ O.D. tubing rather than the expected 3/8″ O.D. tubing. Tractors are customarily stingy with their air sup¬ply, taking care of their own needs first. The trailer service line may not be given air until perhaps a second [or more] later than the tractor. Result? The trailer slams into your tractor and pushes you down the road.

The larger service line requires slightly more cubic inches of air to fill it. Brake activation pressure is not available until the line is filled. Since this is not immediately forthcoming, the trailer brakes are slow. The problem is magnified by attaching multiple trailers. Split seconds can be important.

Another example. The brake system of your tractor and trailers is assembled at the factory. We’ll assume that the designers who pieced them together were careful and did things right. 200,000 miles up the road, a relay valve dies and you have to replace it.

Did you specify and get an exact duplicate of the valve you replaced? Chances are that you did not because few mechanics [or truck owners] are picky enough to require that the part numbers match. If the relay valve was removed and you got another one, that’s where the concern ended. In some cases, there can be as many as a dozen different valves to choose from. Never mind crack pressures.

Crack what? Relay and other valves are specifically designed to pass through a certain volume of air under certain conditions. Crack pressure is the air pressure required to trigger the relay valve into operation. A trailer with a high crack pressure relay valve will have significantly slower brakes than the tractor. When they do apply, it could be like you just threw out the anchor.

Remembering that the trailers already have their own air supply in reservoirs [filled by the emergency line, which also causes the spring brakes to release], notice that the trailer service brakes don’t work until air from the blue line activates the relay valve. If your relay valve sticks or is a poorly remanufactured unit containing a non-spec spring, it will be slow, perhaps slow enough to push you into the ditch.

There are literally dozens of ways to introduce variables into your braking system without half trying — and we haven’t even begun to look at mechanical mutants like aggressive brake linings and mis-spec’d slack adjusters. Combining several uncontrolled variables multiplies the problems. Correctly specing and repairing brake plumbing requires patient thought.

When Specing New Vehicles

When you purchase new tractors and trailers, keep the issue of brake balance and timing in mind. Specifically, this means matching your tractors and trailers to each other, with a bit of leeway to permit matching them with your current equipment.

This is a good time to consider standardization for your fleet. The braking situation is out of control when you have a duke’s mixture of trucks with every imaginable flavor of brake system. A case in point is that every trailer will have different crack pressure requirements so that it will be impossible to spec a tractor to smoothly stop them all.

Ideally, you want your trailers to react quickly and precisely to brake applications. You generally require valves with a low crack pressure; the one to three pound range is desirable. Consult your manufacturer’s factory reps for assistance. Trailer brakes should apply and release almost simultaneously with the tractor.

With your trailers standardized, you can then move on to the power equipment. It is possible to incorporate foot valves and relay valves in the tractors that will channel the appropriate amount of air to the trailers. Tractor brakes can be timed to eliminate deadly delays while waiting for the trailer brakes.

This is a good time to take another look at the current generation of computerized anti-lock braking systems. If these systems perform as intended, they will greatly reduce wheel lock-up and skids.

One caution. Never attempt to mix wedge brake trailers with s-cam tractors. Wedge brakes require more air pressure to achieve the same braking power and can be very difficult to coordinate with an s-cam equipped tractor. Choose one or the other and stick with it throughout your fleet.

The Old Stuff

The next trick is to make your existing equipment work with the new trucks and trailers. You don’t want to unwittingly develop two independent and incompatible fleets. Again, careful thought is required.

Retrofitting may become necessary. You may discover that it is not too difficult to upgrade older trailers to the new standards. A retrofit can involve replacing relay and quick release valves and matching brake chamber sizes to the new requirements. The increase in safety potential could justify the expense.

Actually, this may not be too bad because relay valves are not that expensive. You can specify a particular [and easily obtained] quick release valve and relay valve, then convert to your designated standard system as each old trailer comes due for its PM.

One note of caution: When you decide to re-plumb a vehicle and alter it from its original construction, consult the manufacturer. Their technical people can advise on upgrades to maintain the safety of the rig. They have resources you don’t have and can steer you clear of trouble. Call on them.

On the Road

Looking back to the scenario of the roadside breakdown, how do you handle this? Normally, you will take whatever you can get to be on your way. As we have demonstrated, this is not always a good idea because the components need to be matched for safety and optimum performance.

Most brake valves, both new and rebuilt, have a little aluminum tag attached under one screw. This tag has a detailed part number. Your first action, once the valve is confirmed as defective, is to specifically instruct the mechanic to replace it with an exact match.

If, because of availability problems and time constraints, you can’t get an exact match, you may be forced to take what you can get. In this event, don’t forget what happened. As soon as the truck gets home, replace that valve with the correct part. You can justify the extra expense as an investment in safety.

Ongoing maintenance is important. This means keeping the air system clean and dry to reduce the likelihood of sticking relay valves and premature failure. It also reduces the incidence of freeze-ups. An air dryer incorporating a filter can do this job.

If your compressor pumps too much oil [most seem to pump some after a while], it is time for a replacement. An excessive amount of oil overwhelms the air dryer, then may migrate throughout the system. It congeals at low temperatures to form a sludge that jams valves.

Staying in Balance

Your net concern with the air system is to coordinate every component so they all work together. Keep in mind crack pressures, air supplies, and the mixing of old and new equipment. Combine the factors of careful speci¬fications, thoughtful maintenance, and determined follow-up to increase your braking safety.


  • This field is for validation purposes and should be left unchanged.
Related Articles