The brake pedal fails for various reasons. If you collect the experience of motorists, then you can accumulate at least 12 such reasons. Here they are all considered according to the following principle: how they manifest themselves (there are different failures), why, where to look to identify the problem or eliminate it, and how to restore the brakes without the help of specialists. You will also learn a little about the device and the principle of operation of the system. In the appendage, get three tricks that allow you to stop the car when the brake pedal fails.
The device and principle of operation of the braking system of the car
This is where you should always start. To make a computer work, you must first install an operating system on it. And in order for the head to work, it is necessary to put at least basic information about the problem into it. This is precisely the device and the principle of operation. Knowing and understanding them, you will understand a good half of the reasons for the failure of the brake pedal discussed here without additional guidance.
The classic hydraulic brake system of a car without any bells and whistles consists of the following components and parts:
- Brake mechanisms - located on the wheel hubs, equipped with brake actuators that press the pads against the discs or drums.
- The main brake cylinder is located under the hood, and is a device with four compartments filled with brake fluid and pistons that “press” this fluid to the wheels.
- Lines or circuits - a system of pipes through which the working fluid flows from the master cylinder to the brake mechanisms.
- Vacuum booster - located between the master cylinder and the brake pedal, designed to increase the foot force applied by the driver during braking.
- The brake pedal is the brake control.
- The expansion tank is a reservoir for filling, topping up and controlling the level of the working fluid, located directly above the main cylinder.
Naturally, this is a greatly simplified description of the brake system. But it is enough with his head to understand why the brake pedal fails, and where to look for the cause.
Principle of operation
The brake system of the car functions to ingeniously simply. To slow down the rotation of the wheels, the brake pads are pressed against the discs or drums. As a result of the action of friction forces, the motive energy that the motor has developed is converted into heat and dissipated into the surrounding space. And where there is no energy, there is no movement.
To press the pads against the discs or drums, a brake mechanism is used. It is implemented on calipers. Each of them has a cylinder with a piston, which, under the influence of a working fluid coming under pressure, pushes the pads. When the fluid is not supplied, the pads are unclenched under the influence of the springs, and braking stops. By adjusting the pressure of the working fluid, we can dose the intensity of braking in a wide range.
In order to supply the working fluid to the brake mechanisms at the right time, and even under high pressure, you need a master brake cylinder. Brake fluid must be in it at all times. The expansion tank is responsible for this. If for some reason there is not enough liquid, the system starts working with air. And this is the main reason why the brake pedal fails.
The whole principle of the hydraulic brake system is based on the fact that the fluid does not decrease in volume under pressure. That is, if it is “pushed” through the tubes with a certain force, then the same force will be transmitted to the actuators. If air, which is a gas and not a liquid, enters such a system, problems begin. The gas is significantly reduced in volume under pressure. This means that the force transmitted through the system decreases or disappears altogether, not reaching the brake mechanisms.
The main brake cylinder is actuated through the brake pedal in the car. The driver presses on it, the force is increased by a vacuum booster and transferred to the pistons. Well, they are already pushing the working fluid along the contours to the wheels. Strictly speaking, the described system may well work without a vacuum amplifier. It is needed only for the reason that the muscular strength of the driver is not enough to create the necessary pressure in the master brake cylinder. But we will return to this node later. In the section about myths from couch "experts".
Different types of brake pedal failures
Now, in general terms, consider the varieties of our malfunction. The fact is that the brake pedal can fail in different ways, and this must be taken into account when looking for the cause. It is also important to take into account the circumstances under which failures occur. This helps to quickly eliminate breakdowns that have nothing to do with individual situations.
Types of brake pedal failures:
- with the engine running;
- after replacing the pads;
- after pumping the brakes;
- sharp dips;
- smooth dips;
- the pedal fails sometimes;
- the problem disappears after a few clicks;
- Pedal drops and no fluid comes out.
If the pedal fails when the engine is not running, then this means a complete lack of brakes and an upcoming major repair. Such a breakdown is “good” in that it is not difficult to find its cause. Usually, it is somewhere in a conspicuous place, and is immediately detected upon a cursory examination. But, as a rule, this does not happen very often. Basically, the brake pedal fails when the engine is running.
Failures after replacing pads, fluid and other manipulations with the brake system are separate stories. Each of them is given attention below. If it was possible to correlate the malfunction with a recent repair or service of the braking system, this will be an excellent clue when looking for the cause.
Sharp dips, smooth, intermittent and disappearing after a few clicks - all this does not help much to find the problem. However, it allows us to estimate the scale of the disaster. For example, if the pedal fails only occasionally, this indicates that the breakdown is not critical, and it will be more difficult to find it. Sharp dips, on the one hand, are more frightening, but on the other hand, they simplify diagnosis.
The last common type of system failure is when the brake pedal sags but fluid does not escape. That is, the tightness is not broken. What these types of symptoms indicate is also discussed below.
Why does the brake pedal fail
We talked about the basic information, now let's move on to the specifics. Let's start with a kind of cheat sheet, which you can later use while at the car. Rewrite it on a piece of paper, take a picture on your phone, or, if you are reading this article from it, save the page to your bookmarks.
Causes of brake pedal failure:
- Brake fluid leak.
- Air in the system.
- Filled with poor quality brake fluid.
- Defective brake master cylinder.
- The brake fluid has not been changed for a long time.
- Large gap between brake pedal and booster.
- Large clearance between pads and drums.
- Hub malfunctions.
- Wear of brake pads.
- Overheating of the brakes.
- Non-lapped parts of the brake system.
- Not pumped brakes.
Now let's go through each item of our cheat sheet. In particular, we will consider what the essence of the cause is, how it manifests itself, where to look for it, and how to eliminate it. Even if self-repair seems scary and impossible to you, you will know what to say to a specialist. And he, in turn, will not have a chance to impose on you, non-existent breakdowns that require expensive “repairs”.
Reason number 1. Brake fluid leak
The essence of this reason for the failure of the brake pedal is very simple. If the volume of liquid in the system decreases, it is replenished with air. As a result, you try to brake, and the master cylinder begins to compress the air in the system. Knowing the principle of operation, you now understand that your efforts do not reach the brake pads. The pedal falls off. The car doesn't stop.
The first thing to do is look at the brake fluid level in the expansion tank. If it falls below the corresponding mark, you have already solved half the problem. It is much more difficult to find where the brake fluid has gone. If pedal failures were found in the same place where the leak occurred, it can be localized by puddles or spots under the car.
If you have moved away from the place, then you should carefully examine the details of the entire system. Brake fluid is very "greasy", and leaves well-marked traces. Look for them, first of all, should be near the wheels. On hubs, calipers, matching pipes. Then look under the hood. If the leak is there, then it will most likely be found on the master brake cylinder. It happens that the expansion tanks are leaking.
If a problem is found, it is solved by replacing the leaking part. Don't forget to bleed the brakes afterwards to remove air from the system. Most often, working brake cylinders flow, then the tubes become unusable due to old age and adverse conditions. Under the hood, leaks occur due to bad seals. They either dried up or were killed by low-quality brake fluid. But more on that later.
Reason number 2. Air in the system
At first glance, it may seem that this reason is not much different from the previous one. But it's not. The fact is that air in the brake system does not always mean a leak and a lack of fluid level in the expansion tank. It also happens that air is formed in the lines in the form of traffic jams. When you look at the reservoir, the brakes don't work. That is, no pressure is created inside. The air fills a large volume, and the liquid in the tank occupies a normal level.
However, when you press the brake pedal and it falls through, that's when the air locks are triggered. They shrink under pressure, decreasing in volume. As a result, the force generated by the master cylinder is reduced. Often to such an extent that nothing is left for the pads to actuate. The system, with all this, can remain absolutely hermetic. It is clear that such an alignment greatly complicates the search for the cause.
On the other hand, if the liquid level in the tank is in place, and the pedal is wadded, then this, first of all, indicates just air. The problem is solved by pumping the system. It is possible that this operation was carried out recently, but not of sufficient quality.
Reason number 3. Poor quality brake fluid
90% brake fluid consists of glycol components. The rest is the so-called additives. They are needed in order to protect the system from destruction from the inside. So, if the liquid is of poor quality, these same additives may either not be there at all, or their correct concentration may be violated. In addition, they are not eternal, and eventually lose their original properties.
But back to low quality. Motorists who encountered such a problem talked about how the rubber hoses of the highways delaminate and swell from the inside. Over time, this leads to their destruction and loss of tightness. It all boils down to reason #1 on our list. Fluid drains and air takes its place.
Actually, not only rubber pipes can suffer from low-quality brake fluid. This material is also used for the manufacture of sealing cups in the brake master cylinder. It is clear that their swelling and delamination does not lead to anything good. Over time, the tightness will be broken, the liquid under pressure will leave, and the pedal will begin to fail smoothly. The effectiveness of the brakes, in this case, of course, will suffer.
Reason number 4. Defective brake master cylinder
There are not so many breakdowns in this node. We have already identified one of them above. In addition, natural wear of cylinder parts is not ruled out. In some cases, it can be restored if you can find suitable repair kits. Most often, the master brake cylinder is simply replaced with a new one, or a used one, but in working order.
Calculating this cause of brake pedal failures is not always an easy task. Of course, if it is clearly flowing, then it can be seen with the naked eye. But as for wear, it can only be determined after dismantling and complete disassembly of the assembly. The latter method, as a rule, is resorted to already when nothing else helps to eliminate brake pedal failures.
Reason number 5. Brake fluid has not been changed for a long time
According to the recommendations of most car manufacturers, the brake fluid should be changed to fresh every 40,000 kilometers. Not all car owners follow this rule. It is understandable. Such a run is not a guarantee that the liquid will instantly lose its properties, and the pedal will begin to fail. For some motorists, it does not change even after 100, and after 200 thousand.
In addition to car mileage, there are other ways to understand that it is time to change the brake fluid. The very first sign of its “aging” is the loss of transparency and the acquisition of a dark color. This happens because of the so-called hygroscopicity. That is, it gradually gains moisture, which "kills" the original chemical composition. The pedal from this may not fail, but it will do so due to corrosion of the system parts. By the way, regarding water, and that it allegedly compresses more than brake fluid, it is said below in the myths from the couch “experts”.
Reason number 6. Large gap between brake pedal and booster
Everything is extremely simple here. If the specified clearance is excessively large, by pressing the pedal, you simply do not transfer all the effort put to the master cylinder. To identify such a reason is even easier than all the others. Firstly, it usually does not occur if nothing has been done to the brakes before. Secondly, the brake pedal fails, but the fluid does not go away. Thirdly, due to the large gap, the brakes work later than expected, which, with some experience, can be felt with your foot.
How the problem is solved depends on the design features of a particular car. In addition to adjusting the gap, one should not forget about cases when it is completely absent. And the pedal fails in the described way due to mechanical wear of parts. Accordingly, their replacement will help eliminate failures.
Reason number 7. Large clearance between pads and drums
If the car has drum-type rear brakes, it is worth working out this possible reason as well. Its essence is as follows. When there is an excessively large gap between the pads and the working surface of the drums, contact between them begins with a delay. As a result, part of the brake system at certain points runs idle. As a result, the pedal fails.
It is clear that this cause is not accompanied by fluid leaks and other possible symptoms discussed here. Calculating the problem and its nature is also usually easy. This is observed when the pads are worn, skewed or weakened. The clearance adjustment depends on the design of the brake system and is usually described in the owner's manual, if available. There is no universal algorithm, therefore we will not say anything about this.
Reason number 8. Hub malfunctions
The most common malfunction in this direction is the loosening of the hub fasteners. In this scenario, brake pedal failures can be observed in corners. This happens due to the fact that the loose hub warps and pushes the brake pads apart. If the latter are also rather worn out, then the problem intensifies.
Finding and eliminating this cause is easy. To do this, you need to hang out all the wheels in turn, remove the disks and walk through the remaining parts with your hands. If something has weakened somewhere, it is easily determined. The repair is so simple - tighten, install the wheels and that's it. It happens that when checking the part dangles, but the fasteners seem to hold well. In this case, on the face, as they say, wear. This is where replacement helps.
Reason number 9. Brake pad wear
Not always worn out pads manifest themselves in the form of brake pedal failures. But it is impossible to exclude this reason from the cheat sheet. The essence of the problem should be obvious. To "get" to the working surface of the discs or drums, thin worn pads have to travel a long distance. Accordingly, they overcome part of this distance idly, which is why the brake pedal falls under the foot.
In this case, the brakes themselves do not disappear, but only start to work later. This allows you to understand, without unnecessary gestures, almost from the first time, what is the reason for the failure of the brake pedal. It becomes wadded or excessively soft only at the beginning of its course, and then it works as it should. If so, then pay, first of all, your attention to the degree of wear of the pads. Do not forget that there are two of them on each wheel. Often people forget to look at the block from the inside. But it does not have to wear out exactly the same as the external one.
Elimination of the cause is obvious. Replacement pads. It is better to change the whole set at once. After replacing, do not forget to pump the brakes. Sometimes this is not absolutely necessary. But only sometimes.
Reason number 10. Brake overheating
This happens infrequently, but, as an option, it cannot be ruled out either. The easiest way to overheat the brakes is when driving in city mode on congested roads. Each car has its own limit in this regard. It all depends on the design, the quality of the materials, how the cooling is arranged, and so on. It is clear that it is irrevocably impossible to eliminate such a cause of failure of the brake pedal. From time to time, she will still show herself.
On the other hand, overheating of the brake system is not a malfunction. That is, you don't have to do anything. So, if the pedal suddenly starts to fail every other time, pay attention to the driving mode. It may be worth either driving less aggressively, or stopping altogether until the car is completely without brakes. Attention! It is strictly forbidden to check the temperature of the brake system by hand. Burns are guaranteed. Even the rims should not be touched, since the cooling of the brakes is carried out partially and through them.
By the way, due to overheating, in addition to pedal failures, there may be a hum of brakes.
Reason number 11. Non-lapped parts of the brake system
When the brake pedal fails after replacing the pads, additional vibrations may be observed. In other words, there is a noticeable recoil in the foot when pressed. Often it is more like a vibration in character. Fortunately, the symptom is not a malfunction. But on the condition that you really recently replaced the pads. If not, then you should look for the cause of pedal failures among other options.
As for the described symptoms, they disappear on their own. How fast depends on how much braking is used. Usually, they talk about 500 kilometers. But this is not something that is approximate, but does not mean anything at all. So, if you drive this distance on the track, pedal failures, of course, will not disappear. When driving in urban mode, parts can wear out even in the first hundred kilometers. By the way, it also depends on the quality of the pads themselves.
Most importantly, if brake pedal failures were observed after replacing the pads, then this should not be forgotten when the problem goes away. As a rule, upon completion of grinding, the level of liquid in the expansion tank drops slightly. This moment is desirable not to click. The fluid needs to be topped up, and ideally, the brakes should be bled again.
Reason number 12. Not pumped brakes
This reason for the failure of the brake pedal could no longer be painted separately. After reading, it should be clear what the air in the system leads to. However, pumping is often neglected or not done carefully enough. Therefore, let these couple of lines be to you as a reminder. After any manipulations with the brake system - pump it. It's easy, not very long and free if you do it yourself. The only difficulty is that you need an assistant.
Two myths from couch "experts"
Since myths are not very helpful in solving the problem of brake pedal failures, we will not particularly describe them. However, it is desirable to know about them, since the two reasons mentioned are often attributed to the problem in question. Some do this because of a misunderstanding of the principle of operation of car systems. Others thus try to impose non-existent problems. Others just decided to be clever.
Myth one. The brake pedal fails when the engine is running due to a broken vacuum booster membrane. Actually, it is not. When the membrane breaks through, the pedal does not become cottony. Rather, on the contrary. She becomes incredibly tough. Just like when the engine is off. Brakes, at the same time, do not disappear anywhere, in contrast to the cases discussed above.
Myth two. The water that has entered the system is more compressed than the brake fluid, and because of this, the brake pedal can fail when braking. Actually, this is nonsense. The compressibility of all liquids is about the same. At least not so different that it somehow affects the volume under pressure in the car's braking system. Of course, the presence of water in this case is bad. But not because it supposedly shrinks more. The pedal fails because of it for other reasons discussed above.
The brake pedal fails - how to stop the car
In the arsenal of any driver, these three methods are simply required to significantly increase the chances of avoiding a tragedy in an emergency. The only "but" - simple knowledge will not help. Need a workout. Therefore, without waiting for the next brake failure, work out in advance in practice the proposed techniques for stopping the car (besides the last one).
The first way is engine braking. With this technique, in theory, you should be familiar with driving school. If for some reason you do not know this method of controlling the machine, we will briefly consider the essence. Engine braking consists in slowing down the car with the help of the resistance of the low gears of the manual transmission. In practice, everything is simple - the clutch pedal is depressed, the lowest possible gear is engaged, then the clutch returns to its original position. At the same time, we do not touch the accelerator. Albeit not as effective as with regular brakes, but the car will slow down. Already this is sometimes enough to avoid trouble, or at least minimize its scale.
The second method is braking with a parking brake. Attention! The method is extremely dangerous if used ineptly. You can make it even worse than it was, as there is a risk of blocking the rear wheels. However, learning how to effectively stop a car with just the parking brake is quite realistic. To do this, you do not need to pull it, as they show in films about racing. Confident, but smooth retraction of the handbrake is more effective than engine braking. So, it makes sense to practice in advance on a site free from people and cars.
The third method is contact braking. The method is not pleasant. But when there are few options, then why not. The essence of the method is to try to stop the car by bringing its side parts into contact with surrounding objects. It can be a fence, a wall, and so on. By the way. This is the only one of the three ways that you should not work out without need, for the sake of training. Just know that this can also sometimes get out of the current situation.
As a result, we will probably not incite what has already been said. Let's just add a few tips. Firstly, if you suddenly find that the brake pedal has begun to fail even a little bit, further operation of the car is mortally unsafe. Secondly, braking is far from the only way to avoid a collision. There are situations when it is better, on the contrary, to step on the gas. Proven in practice.