In operation, when the tow vehicle slows, the inertia of the trailer presses the actuator against the coupler, pushing the piston into the master cylinder. Like stepping on the brake pedal in a car, this applies pressure through the brake lines to the individual brakes at the trailer wheels. The system self-regulates the amount of braking force because the force on the piston is proportional to the rate of slowing of the tow vehicle.
The more the tow vehicle tries to slow, the more force is applied to the piston and through the hydraulic fluid to the trailer brakes. Conversely, when the tow vehicle accelerates, the hitch ball pulls the coupler forward, extending the piston and releasing the brakes. A very simple, cool design.
However, an unintended consequence of surge brake systems is that the actuator also applies the brakes when backing up because the tow vehicle is pushing against the inertia of the trailer. On flat ground with a light trailer, this may not be a problem. However, it can be a big problem when backing up a hill or driveway with a heavy trailer, especially if the trailer is equipped with disc brakes instead of drum brakes. Therefore, the brake system requires a method to deactivate the brakes when in reverse.
Some actuators allow the driver to deactivate the brakes manually by temporarily inserting a lockout pin into the actuator to keep the actuator/coupler mechanism from compressing. However, the pin will prevent brake actuation both in reverse and in the forward direction. This presents a serious safety hazard because if the driver inadvertently leaves the lockout pin in place while driving forward, the trailer brakes will remain deactivated.
A better approach is to use a system that automatically deactivates the brakes only when backing up. In this respect, drum brakes have an advantage over disc brakes because the design of the brake shoe mechanism in drum brakes leverages much of its braking force from the forward rotation of the drum. As a result, drum brakes are very ineffective in reverse, so backing up is not difficult even if the brakes are applied. In addition, some drum brakes utilize a "free-backing" design, which virtually eliminates all braking action in reverse.
Disc brakes, however, are equally effective in forward and reverse because the braking force is independent of the direction of rotation of the rotor. To prevent braking in reverse, actuators intended for disc brakes often utilize either a blocking or bypass solenoid. The solenoid mounts between the master cylinder and the brake line, and connects to the backup lights of the tow vehicle. Therefore, the solenoid actuates only when the vehicle is in reverse gear, eliminating the potential of the brakes remaining deactivated when driving forward.
As the name suggests, a blocking solenoid blocks the brake line, preventing pressure from the master cylinder from pressurizing the brake line. However, a blocking solenoid can still result in the trailer brakes activating while in reverse because the solenoid will not relieve existing pressure that can already be in the brake line. This will occur if the driver stops while traveling downhill and shifts into reverse to back up the hill. Even though the blocking solenoid will prevent the master cylinder from applying additional pressure to the brake lines, the existing residual pressure in the brake lines can keep the brakes applied.
As an alternative, a properly designed bypass solenoid relieves the pressure from both the master cylinder and the brake lines by opening a bypass port into the brake fluid reservoir. This prevents the master cylinder from applying new pressure to the brake lines, and relieves any existing residual pressure in the brake lines.
There is a basic flaw with the very concept of surge brakes, which is that there is no way to apply them without braking the tow vehicle. I am aware that they allegedly have a breakaway application device, but that is of no use when you simply need to apply brakes on the trailer, which you need to do non the occasion that the trailer is forced into an oscillation mode, either by the shockwave from a truck passing too close, or a number of other causes. And if your vehicle brakes fail, that surge brake is not available to slow you at all. So the first step is to replace the surgen brake with a system that can work. One "older concept" system uses the tow vehicle's power steering pressure and a control valve piloted by the tow vehicles brake system pressure to activate a hydraulic cylinder on the trailer that drives a secondary master cylinder. That system can have an electrical valve to provide remote operation of the trailer brakes, for emergency and parking use. Other systems have used tow vehicle engine vacuum to drive the trailers master cylinder. Of course, all of them cost more than a surge brake system, but all of them beat driving without any brakes.
I have stopped nmy van, while pulling a travel trailer, using the trailer brakes alone. It is not nearly as good as the four-wheel brakes were, but I did get stopped. That was when a front brake line bust on my 1985 Dodge van. Contrary to the claims, when you lose front brakes, you have no back brakes either. At least, not on a Dodge from that model year.
I still have a boat, but have not used it in years. I am about to buy a new truck and want trailer braking.....I never thought about what you mentioned ...thanks!
Warren, Ann and tekochip: Here's some more info about the driving conditions I was experiencing when the trailer brakes started smoking. That day I had only driven a short distance with the trailer, all on level roads. Before entering the freeway, I had driven less than two miles from home through town at a maxim speed of 35 mph. When I entered the freeway, the brakes started smoking within the first 500 yards. I was not using the brakes because I was just getting up to freeway speed.
Regarding riding the brakes, some interesting dynamics occur with surge brakes. In steep downhill situations, controlling the speed of the tow vehicle by using a low gear and not riding the brakes does not avoid the potential for the trailer brakes to overheat. Regardless of how the speed of the tow vehicle is controlled, if it creates a great enough resisting force against the trailer inertia, the surge actuator on the trailer will apply the trailer brakes.
One reason I was so surprised with the overheating was that just the week before this problem occurred I had towed the boat on a 500 mile trip. That trip included mountainous roads and 100 degree air temperatures. Although there were problems with individual brakes on the trailer, there was never any problem of all four wheels smoking. Interestingly, this trip included going down "The Grapevine" grade on Interstate 5, the main north-south highway in central California. This grade is about 6 miles long, has a 6% slope, and includes escape ramps for runaway trucks whose brakes fail. Despite the potential for the trailer brakes to overheat, the brakes appeared to handle it fine, with no smoking.
tekochip, that's a whole 'nother subject that many drivers have complained about here: tight curves on exit ramps, as well as very, very short exit ramps in some places. OTOH, when driving in New England, especially Massachusetts, many years ago, I was amazed at how poor the freeway exit signage was in letting people know what exits to take to get to specific places, (XYZ village name instead of Highway XX North, Highway XX South). It reminded me of some areas of Britain: you had to be a local to understand where you were going and which road took you there. The last time I drove there, in the early 2000s, I did see some improvement.
You're right about the roads in California. I'm from Chicago and my first time driving in California I was shocked by how tight the turns are on the exit ramps.
Warren, no offense taken; it was clear you don't drive the kind of roads I do. Those "Deadman's Curve" labels are not symbolic or imaginative, so my luck was actually quite good. The big problem I see is not enough warning symptoms before failure occurs, as well as brakes not designed to handle the amount of use required on long descents, no matter how much you use lower gears. We've got deer, too, up here and one took out the entire front end of my first husband's 1965 Mustang, which had a V-8 and a heck of a lot of steel in it. I bet you've got some great mud stories, though--and that's *not* sarcastic.
Warren, the first time I was young, inexperienced on mountain roads, and alone. I didn't know how to use lower gears, even on an automatic (both cars were automatics). In both events, only one of the brake sets went out, which is why I live to tell the tale. The second time I was highly experienced and not alone--fortunately, the guy who was about to be my husband said not a word and let me do my thing--and people had died on that slalom-like section--so we lived. In both cases, there had been no warning signs of any kind, such as brake noise, and the second car was well maintained. I know several people who've had similar experiences, more than once, and Highway 1 has more than one "Deadman's Curve" sections (title of a Beach Boys song way back). Perhaps your mountain roads aren't as steep or as windy as they are in the Coastal Range in California.
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