INTRODUCTION: Extending a motor home’s air brake system to provide air over hydraulic brakes for a towed vehicle (toad) is an excellent way to achieve a dependable, low maintenance and easy to use toad brake system. However, the perceived risk of dangerously compromising the coach’s air brake system by tying the toad system directly into it is a major impediment. This article presents a system design that allows directly extending the coach air brake system to a towed vehicle without creating a serious risk of compromising the coach brake system, even if the toad system fails. It is modeled on Federal Department of Transportation (DOT) truck safety regulations.

BACKGROUND: My toad is a full size, 4WD pickup that weighs 5200 pounds. When I picked up the coach, I borrowed a neighbor’s self-contained brake unit for the return trip home. It is a ‘black box” inertial type that pushes the toad’s brake pedal when the pendulum control mechanism senses slowing (or a downgrade?). There is no proportional braking. The unit either acts with full force, or it is off. It is powered from the toad’s cigar lighter. We were in flat country the first two days we used it. I could feel the toad pushing me slightly when I would slow and turn at the same time, so I increased the sensitivity to activate it earlier in the braking cycle. The third day we were in hilly country with storm conditions all day. When we stopped that evening, the toad battery was completely dead. I think exhaust brake application and the tilt due to highway downgrades activated the unit so that it constantly pumped the brakes all day long. I subsequently discovered that I could not feel any difference in the way the coach handled with or without the device.

My experience with the self-contained brake, the inherent inability to exercise any control over the unit and the lack of proportional braking capability caused me to dismiss this type of brake system from further consideration. Research on available systems lead me to Bob Gummersall’s article, “Toad Braking Systems”. I subsequently purchased an air over hydraulic master cylinder kit and a breakaway emergency actuating kit.

After receiving the brake system kit and seeing how it was to be installed, it occurred to me that there was a risk of losing the coach's brake system while underway if the air line between the coach and toad were severed. It also seemed that the extra air load on the rear brakes could impair effectiveness of the coach brake system. Finally, being unaware of the Magnisson-Moss Act, I was concerned about voiding the warranty on the braking system and consequently voiding my insurance in case of brake failure. The latter is what gave teeth to my perception of the risk. I decided not to install the system I had bought in the way recommended by the supplier. Instead, I needed to find a way to reduce the risk level to what I considered insignificant.

A simple volume ratio estimate suggested that tying into the rear brakes on the coach would retard their application compared to the front brakes, but only by an insignificant fraction of a second (less than one wheel turn at 60 mph).

Review of the Magnisson–Moss act suggested that no warranty problems should exist provided that I did nothing that could be considered egregious.

Since I knew almost nothing about air brake systems at the time, I undertook a research project to acquaint myself with the mechanics and the underlying design principals. It took some time, but finally the obvious came into focus. Safe air brake systems similar to what I wanted are all around us all the time on the highway. They are the brake systems used on truck-trailer units commonly referred to as “semi’s” or "18 wheelers". The technology for minimizing the consequences of breaking the lines between tractor and trailer is well established. It is written into Federal regulations. Component parts required are readily available at reasonable cost.

SYSTEM OVERVIEW: The basic system described in this article is essentially the same as systems used for truck semi-trailer braking systems, except that brake control equipment normally installed on a semi-trailer is split between the coach and the toad.

The toad system ties into the supply and service (brake) air lines at the rear axle of the coach, and operates as a part of the rear axle brake system. Two air lines are required between the tie-in point and the rear of the coach: supply and service air. Technically, these additional lines increase the risk of system failure simply because they add to the length of air line already installed. However, a snug fitting cover of “loom” protects the lines (loom is a corrugated plastic material that is split to allow insertion of the air tubing), and they are routed inside the frame of the coach. Any increase in the probability of system failure due to adding these two lines is considered to be vanishingly small.

At the rear of the coach, the supply-air line is connected to a “Pressure Protection Valve.” It shuts off supply air to the toad system in case air supply pressure is below 75 psig (or as set between 70 and 80 psig). That is above the minimum pressure required for satisfactory coach brake performance. The pressure protection valve feeds supply air into a “Relay Valve” and a 20-foot length of half-inch brake tubing (coiled into a small circle). The tube is used for toad air supply storage in lieu of a large, cumbersome steel air tank. A bonus is that the storage tubing also provides a connection for utility air at the rear of the coach.

The service-air line connects directly to the relay valve. It provides a signal pressure, which causes the relay valve to pass supply air to the toad at the same signal pressure. Since the coach’s service-air line ends at the relay valve, it cannot be affected by damage to the toad air line.

A short service-air line under the coach hood connects the relay valve to a quick connect air fitting mounted at the base of the rear bumper. The air line in the umbilical to the toad connects at this point. The umbilical consists of a commercially available coiled four-wire electrical cable (lights and signals) with a piece of loomed quarter inch air tubing inserted through the center of the electrical cable. The umbilical air tube has a male fitting on the end that mates with the coach quick connect, and a quick connect on the other end where it connects to the male fitting on the toad. This set up allows the coach brakes to function properly with the umbilical either removed from or attached to the coach. Note also that this allows the air line on the toad to remain open when not connected to the coach. The air master cylinder breathes when the toad brake pedal is moved. Closing it would interfere with normal braking while driving the toad.

The toad air connector is shielded to keep out trash and to minimize dust intrusion into the air line. I used a small fender washer with a stove bolt to plug the connector. Threads are ground away on two sides of the bolt to prevent escaping air from blowing the bolt out of the connector when the toad brake pedal is released. A modified large paper clip keeps the plug in place while driving. It is easily removed.

A breakaway switch, with appropriate controls, is installed on the toad to stop it quickly in case of separation from the coach. A small emergency air storage tank is installed under the toad’s hood. Holding the coach brake pedal down hard until coach air pressure builds up charges it. An electrical jumper was added so the toad’s horn sounds continuously if the breakaway switch is activated. Conversely, honking the toad’s horn triggers the breakaway valve, and discharges the emergency air tank. This would normally happen only during hookup, so it is not a significant problem. It is easy enough to remember to recharge the emergency tank before driving away.

An air pressure regulator is installed between the breakaway control valving and the master cylinder to limit maximum braking pressure on the master air cylinder. The pressure regulator functions as a rudimentary Antilock Brake System to stand in for the toad’s ABS when the ignition is off. As with any ABS, it prevents wheel lockup on the toad, as might otherwise happen during panic stops or breakaway. That provides maximum control to the coach driver, and minimizes the possibility of jackknifing the toad. Preventing wheel lockup also minimizes stopping distance.

There are a couple of caveats that anyone installing a toad air brake system such as this one should know. The first is that the brass tube fittings (compression type) used on air brake systems are different from the garden variety available at the local hardware store. The caps are not interchangeable. Use only the stronger, DOT approved fittings. They are commonly available at truck and fleet supply stores.

The second caveat is that the service-air tie-in at the rear axle relay valve on the coach absolutely must not be made between an ABS modulator and its brake pot. Such a connection would unbalance the rear brakes at best, and could destroy their effectiveness. Most DP chassis today are equipped with antilock brakes. The brake modulator is often bolted directly to the relay valve, thus eliminating the possibility of connecting the toad service-air line between the relay valve and the modulator. There is normally a service-air port on the relay valve that feeds the spring brake “Anti-Compounding” valve. Tee off of this port to supply service air to the anti-compounding valve and to signal the toad relay valve at the rear of the coach.

PERFORMANCE SUMMARY: The system is installed on a 38-foot diesel pusher (DP). It has traveled over 6,000 miles now, and has worked as intended. On at least two occasions when sudden, unexpected traffic events required a near panic stop, the much shorter stopping distance made possible by the toad brake system prevented an otherwise probable collision. The system has also twice proven that a broken toad airline does not adversely affect coach brake performance.

The first failure test occurred during the installation process. In my haste to keep service appointments for the coach, I forgot to replace the air line between the toad relay valve and the bumper (I had installed it, and then later removed it). I drove some 30 miles around town in stop and go traffic (without the toad), not realizing that the toad brake line was “open”. Transmission, front-end alignment and brake-adjustment service technicians also drove it into and out of their respective shops. Coach braking performance was unaffected. Only when I arrived back home and had to exercise the brakes frequently to park the coach did I note that air pressure was a bit lower than usual. I realized that the system had been “tested” only when I went to finish the installation. (I am told this is called “Flat Forehead Syndrome”.)

The second test was under actual travel conditions. The same air line was left too long when reinstalled, and was not adequately secured near the coach engine. It came into contact with the fan belt, which promptly sawed a hole in it. I discovered that the toad brakes were not working when I felt the toad pushing the coach as I pulled into an Interstate rest stop. Again, the coach brakes were not significantly affected.

ACKNOWLEDGEMENTS: I must mention that I have heard from several people who have the master cylinder kit that I purchased installed on their units in the manner recommended by the manufacturer. Some people have over 30,000 miles of experience with the system. All report being satisfied with its performance. I have also been told of tests that simulate toad air line failure using the manufacturer’s installation method. Results reportedly show toad air line failure will drag coach air pressure down, but should not cause a catastrophic failure of the coach brake system. Ultimately, the installation method used comes down to how much risk the user is willing to accept. That assumes the user is aware of the risks.

My thanks to Bob Gummersall for his article “Toad Braking Systems” that started me on the path to developing this system, and for his encouragement to write this article to document the results.

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