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.
My comment was about the fill cap on the master cylinder. The fill cap on a hydraulic brake master cylinder is vented to the atmoshphere. Water and or moisture laden air can and will get in to the reservior where the fluid will grab the moisture and then proceed to corrode the innternal parts given enough time.
I worked part time for a mechanic, for gas money while I was in school. I had always worked on my own vehicles, but that experience removed all the 'fear of fixing' from me. The problem I have now is that I don't want to do that work anymore, but I have yet to find a 'professional' mechanic that does work to my standards.
You are very correct that you must plan out the repair. You need to be prepared to spend the time and money to do it right or it will be pointless for most cases.
The air vent is closable and used only for bleeding air from the system. If it were left open, the brakes would not work. Hydraulics are necessarily a closed loop system.
Water infiltration to the system happens when the brakes are submerged to a depth that applies sufficient water pressure to the outside of the seals. This pressure along with the mechanical motion of the piston action flexing the seal can allow water into the system. The brake components don't go very deep, but the wheel pistons can get down to 5-6 feet when the brakes are set during launch and recovery. But remember that within just 33ft of the water's surface, standard atmospheric pressure is doubled.
Well, yeah, but the right solution is a backup car, not using professional services that may not be as careful and meticulous as you are.
Reminds me of a time when four of us engineers carpooled together, back in the 70s. One individual, Tom, was like you--take the car and TV to "professionals" for fixing. He mentioned his frustration--he had taken his car to four garages and it still didn't run right when it was cold, common in Minnesota where we lived.
Another of us, Curly, said he would fix the car, but wanted to see the invoices so he could see what had been done. He found that the first guy had replaced spark plugs and ignition points and condenser. These were common wear-out parts that had to be replaced every 10,000 miles, and were probably replaced without even checking to see if they were worn. The second guy had also replaced the plugs, points, and condenser. As had the third and the fourth.
Curly concluded that he probably didn't need to even look at those items. He poked around for a few minutes and found that the heater tube which carried hot air from the exhaust manifold to the carburetor to prevent carburetor icing had rusted/burned through right where it exited the exhaust manifold. A $3.00 replacement at the auto parts store and fifteen minutes to install it solved the problem.
As we talked about it, we concluded that all four shops had probably diagnosed and repaired the car inside a warm garage, not in the cold environment where the problem actually manifested itself.
So the question, Tim, is "Do you really want to let a 'professional' do the work, and how many trips back to you care to make?"
Brake fluid is glycol based and quite hydroscopic. Periodic fllushing will clearly show the old fluid as darker (from rust) and the new, fresh fluid is water clear. Old glycol brake fluid with a high moisture contect has a lower boiling point and causes corrosion. Disc brakes can get hot enough to boil brake fluid during heavy use so flushing is specified as part of the normal maintenance cycle. Troubleshooting surge brakes can be extremely frustrating. People don't learn they have a problem with their brakes until they either crash when the trailer brakes don't work, or they have smoke when they don't stop working. Good troubleshooting and interesting article.
As a kid, I used to tear everything apart. Now, as an adult, I have a healthy fear of some engineering missions. It is never a good idea to start troubleshooting your only vehicle for a minor problem on a Sunday when you need to go to work on Monday. This is safer to take to a professional.
I've never seen anybody off-load a boat without backing the trailer all the way into the water. All hydraulic master cylinders have an air vent, otherwise the returning fluid woulld be trapped by a pressure gradient. Water can and will get into a boat trailer brake system. Preventative maintenance is the key to longeveity and reliability in any type of vehicle or equipment. True, it may not be necessary to flush annually on your street driven grocery getter but the same does not hold on a boat trialer. Which, I think, is what we were taking about.
I agree that boat trailers are not maintenance-free.
However, I respectfully disagree with your general statement that all brake systems need to be flushed annually. I am not sure what kind of car you drive, but none of the Fords, Chrysler, or GM cars or trucks I have owned specified hydraulic system flushing as a periodic maintenance item. This is not necessary- it is about equivalent to changing your oil every 500 miles- it can't hurt, but it's just not necessary. If it were, manufacturers would certainly recommend it to avoid liability and provide dealer service departments with additional revenue.
Automotive hydraulic systems can (and do) last for decades without being flushed. Usually the only brake system maintenance that is required is replacement of linings and resurfacing or replacement of drums/rotors. When seals or hoses crack or start to leak, then it is time to replace them and flush the system while the hydraulic system is open.
BTW, bleeding and flushing are two different tasks. Bleeding is usually done to remove air, not to remove all the fluid.
It's been my experience that normal people either pay someone else to fix it or just live with the problem for ever. Fear seems to be the biggest problem most have in never trying to repair anything. I can't tear that apart, I will never get it back together, or I won't be able to find the parts, well this stops almost all repairs right there. Being an engineer, we love to dwelve into the guts of the problem, are never satisfied until we have found the root cause, finding joy only when we got that hidden gremlin exposed. Engineers don't fail, don't break things, and never stop trying, we are only adding to our knowledge bank for the next problem.
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