The mechanical structure of hydraulic disc brakes is more precise and complicated than that of C brakes, so riders are not recommended to replace them on their own except for simple debugging. 

 

Problems commonly encountered with hydraulic disc brakes include strange noises, improper trips, air mixing, oil leaks, overheating, etc. Generally speaking, other than the strange noises associated with calipers, other problems can be described as "oil" related.

 

Over-tightened brakes

Newly purchased bikes or bikes with disc brakes often have the problem of small gaps and tightness. The correct break-in method is: choose a flat and open outdoor road, ride the bike to a speed above 10 mph, brake hard to walking speed, i.e., about 5km/h, repeat 20 times; then ride the bike to a speed of about 15mph, apply the brakes to the end, repeat 10 times, the break-in will be over, and you will feel the braking effect getting better and better. 

 

Brake noise

The problem that causes brake noise is mainly due to improper installation position or the caliper is mixed with mud and foreign matter. We can solve this in order by elimination method.

First, check whether the disc is located in the middle of the two lining pads, which the knob on the caliper can adjust; then, check whether the disc is deformed and deflected. A slight deformation can be corrected or replaced with a new disc; finally, check whether the wheelset is correctly installed. If there is deflection, the wheelset must be reassembled.

If there is no problem in the first step, proceed to the second step: clean the caliper disc and replace the liner plate. First, we can remove the brake lining pads and wipe the pads and discs with a liquid mixture of alcohol in equal proportions. Then dry them with paper towels; secondly, check whether the pads are stained with oil and if they need immediate replacement.

 

Brake system full pressure

Full pressure in the oil disc means that the oil circuit is overfilled with oil or absorbs water vapor, especially DOT brake fluid, which is highly hygroscopic. As time goes by, the pressure of the fluid line that absorbs moisture rises, resulting in smaller and smaller gaps and poor rebound of the lining, which makes it easy for the wheels to rotate poorly and hold when the brakes are applied, causing potential danger.

We can replace the oil. This is suitable for vehicles that have not been ridden for a long time and for vehicles that have been changed more than half a year since the last oil change. The oil quality has dropped significantly, and in this case, the brakes and discs will rub excessively, so a new oil change is more effective; or drain the oil properly. Loosen the oil filler screw at the oil filler hole on the brake handle, and the excess oil will spill out naturally. Wipe the oil with a paper towel to tighten the screw. If the wheel is hard to turn, we can repeat this step until the wheel turns smoothly.

 

Brakes out of fluid and air

Due to the damage to the oil circuit, disc brakes will be short of oil or mixed with air, which will lead to a soft grip and weak braking. Then, we need to expel the air and replenish the oil. At present, except for Shimano hydraulic disc brakes which use mineral oil brake fluid, most other brands use DOT oil. The boiling point of mineral oil and DOT oil after absorbing moisture are both lower (such as DOT4's dry boiling point is 446 ℉, and the so-called wet boiling point is the boiling point after mixing with water. The more water content, the lower the boiling point), resulting in frequent braking under the oil circuit heating up and boiling after damaging the oil pipe, especially the oil pipe connected to the caliper, more likely to leak oil and mix with air.

If there is air in the uppermost part of the oil circuit, you can use a needle syringe to inject the corresponding oil and fill it up. You can choose discs with better heat dissipation using mineral oil disc brakes to avoid boiling and oil leakage.

How do we determine if the oil brake is leaking? An oil stain on the surface of the brake does not necessarily mean an oil leak. We also need to judge whether the oil leak is confirmed according to the actual situation after the oil disc is actuated.

First, we must do some preparation work: clean the hydraulic brake. Wipe off the dust on the handle with alcohol wool. Next, we need to make a preliminary judgment on the clamp end and carefully observe whether there is oil leakage from the four clamp parts.

The first part is to observe the position of the seam of the clamp. If there is oil, it may be caused by the damage or leak to the upper and lower seat seals.

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The second part is to observe whether there is oil leakage from the joint screw patch. Excessive deformation and rupture of the brass sleeve, improper assembly of the oil tube, and slipping of the screws can also lead to oil leakage.

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The third part is observing whether oil is oozing out of the oil leak nozzle. If there is oil, it may be that when locking into the oil spout screw, the teeth seam is stained with mineral oil. After riding or the external temperature is too high, the oil is discharged outward, and dirt is attached to it, mistaking it for an oil leak. 

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In the last part, carefully remove the brake lining and check if there are oil stains on the back plate of the lining. If there are oil stains, it is likely that the piston is leaking oil and needs further determination. We need to use an alcohol cotton ball or ultrasonic cleaning machine to clean the inside and outside of the clamp, then spray the surface of the oil disc with an air gun to keep it clean, install the oiling block and press the brake to check if it is solid. If it feels soft, you need to make up the oil. After making up the oil, you also need to clean up the oil stains to avoid misjudgment.

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If there are persistent bubbles on the edge of the upper and lower seat pistons and oil stains when the block is removed, the clamp must be leaking oil, and the whole clamp set needs to be replaced. If there is no oil stain and no air bubble in the piston area, there is no oil leakage.

Next, we need to determine if the brake handle is leaking. Operate the brake handle 5~10 times and observe if there is any oil leakage from the three screws. Improperly assembled oil groove washers or deformed washers can lead to oil leaks. Continue to operate and observe if oil is coming out of the oil groove cover. Then observe whether oil flows out from the piston part of the handle. Debris on the aluminum piston seal and piston surface can cause the aluminum piston to wear and leak oil. Observe whether there is oil seepage from the handlebar connecting the screw part. If there is oil seepage, it may be caused by deformation and rupture of the copper sleeve or improper assembly of the oil tube. If there is oil leakage in the above handlebar parts, then the whole handlebar set needs to be replaced.

When your bike encounters a problem, it is recommended to be referred to a professional repair store to ensure that your bike's brake system can operate safely. 

January 10, 2023 — Qualisports USA

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