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  Hydraulic Brake - Diagnosis



The hydraulic brake can be a nuisance, of course if there is a lack of maintenance, but even this does not protect against defects. Why? Because the term 'maintenance' mainly refers to visual inspection. and understands certain tests. As a rule, however, this does not mean (partial) dismantling, which would bring even more errors to light.

Actually, a general inspection cannot be held responsible for a brake failure shortly afterwards. After all, the similar brake force at each axis has been tested. The disks and linings were also evaluated, of course only as far as they were accessible.

However, only a snapshot is possible by testing each wheel brake for the same effect. Perhaps so much abrasion has accumulated somewhere that it will soon block a flow. Or a hose is close to its load limit, but has survived the steps taken by the inspector.

Not every brake hose is buckled during the main inspection and thus checked for cracks. Also, leaks are not chalked up as long as they are only present between the cuff and outer cover, thus won't penetrate outward. With the drum brake, the anchor plate can already be oily as long as there is no significant liquid content on the linings . . .

The hydraulic system is a fine thing, especially when it comes to operating forces and uniformity. But woe to leaks. And not even that, even the excessive air content can already significantly damage the system. The usual pressure point of a brake system also has a lot to do with psychology and thus with confidence in the system.

But one thing at a time: The ability to detect a leak depends largely on its size. With a comparatively large amount of brake fluid leaking out, the brake pedal can be adjusted up to a good half its way from the normal pressure point to the bottom plate. Then the other brake circuit will engage and you are rescued.

If the opening in the system is relatively small, only a small amount of fluid escapes each time the brakes are applied. Here the maintenance could give an early signal, because the two parts of the reservoir then have different heights. At this point one is rightly reluctant to use the term 'refill container', because brake fluid is generally not to be 'refilled'.

If something is missing, then something is leaking and simply refilling is of little help, is even negligent. Because here a warning signal was ignored, even whitewashed. So first the cause fix it. The system will probably have to be emptied anyway and filled with new brake fluid afterwards.

Where is the most likely place for a leak to occur? Everywhere where movement is involved. If such a rubber seal is permanently crushed, e.g. at a screw connection, a defect is unlikely. If but when they are pushed back and forth on a metallic surface, things get trickier. If small steps are formed, which are then violently crossed after a certain time, then probably damage is the result.

This is why bleeding with the brake pedal is also somewhat problematic, as both pistons and their cuffs usually reach areas where they have never been before. What do these and the transitions look like? It's like a rusty old-timer engine that you want to get running, even though you have to dismantle it and put all parts into I'd have to put things in order.

And then the hoses, which you need to get to the wheel brake with the line. Every spring and steering movement in the front also involves hoses. Sure, they do this longer and longer, but together with environmental influences, the leakage will eventually be eliminated. Even the durability against pressure can suffer. Then the inner tissue tears and bumps form.

By the way, such a behaviour of pressure hoses is a special deception, because they could break when you need them the most, e.g. during an emergency stop. Here the term 'maintenance' has an additional meaning. One could come up with the really glorious idea of replacing all brake hoses after about ten years.

Brakes, steering and tyres, systems with possibly final effect. Of course you have two brake circuits. But have you ever tried to brake a vehicle from a higher speed by handbrake? This would correspond to a failed front brake with a black-and-white layout. Please do not do it!

Why? Because on the one hand you notice a quite weak handbrake effect and a clearly less than half as good braking effect than usual, but on the other hand you are aware of the danger of a to a blocking rear axle. You may know that rally drivers use handbrakes for drifting that are particularly easy to handle. For normal drivers, the drive can also land in botany.

As already briefly mentioned, a brake system can leak internally. You can operate then the brake pedal very slowly halfway to the floor plate without realizing any brake fluid coming out of the master cylinder. If this happens more often, then it can only be that brake fluid returns quasi unnoticed to the expansion tank.


With the wheel brake cylinder, no matter whether disk or drum brake, the brake fluid must leak at some point. This should be easier to locate with the former than with the latter, because here the drum completely shields the wheel brake cylinder(s) from the outside world. So better a complete analysis: If you can exclude the main brake cylinder or have already repaired it, take a look at the after a clear loss in the corresponding part of the compensation tank, the front disk brake(s) are applied (picture above), in order to then remove the rear drums quite a lot of leaked brake fluid is to be expected (figure below).


In case of apparent tightness, make sure to look behind the visible cuffs.

In principle, hollow tools can be used to machine cylinders and insert new cuffs, but only if the cost is worth the effort in view of the price for new parts. With disk brakes, one would anyway use plead at least one new pair of calipers. It is not only due to incorrect machining that another possible error comes into consideration here, although less often in the meantime, namely the tilting of the piston(s).

This is an extremely unpleasant situation. Anyone who has ever owned a vehicle with such a mistake knows what I'm talking about. There you slowly roll towards a red traffic light and suddenly it jerks briefly and you're stopping without applying the brake. It has been doing its fateful job the whole time and has often worn one lining worse than the other. As a rule then also two new disks due. But the expensive ones are of course the calipers.


Of course, there are other possibilities why a brake doesn't loose. It has proved to be a good idea to drain some brake fluid in such a case. If the disks or drums then rotate again the return flow is disturbed. Then check the so-called piston rod clearance between the brake booster and the primary piston of the main brake cylinder. For certain models, you can remove the expansion tank. If the primary sleeve can be seen through the compensating bore when the brake pedal is not depressed, there is something wrong.

To round off possible faults in the hydraulics, possible blockages, e.g. due to very porous hoses, should also be mentioned. It is astonishing that ABS units are not affected by leaks. Otherwise, during the general inspection all screw connections and also the laying and fastening of the cables will of course be inspected more closely.


First approach for a repair







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