Adjusting and bleeding hydraulic bike brakes
Hydraulic brakes are used primarily on bicycles where precision, responsiveness and reliability are required. First of all, these are heavy, massive mountain bikes designed to overcome steep slopes and climbs. Also, hydraulics have become an integral part of high-speed bicycles, since it allows you to very accurately and quickly drop speed.
The principle of operation of hydraulic brakes is very similar to mechanical brakes, where the brake pads are driven by a cable. The only difference is that in the hydraulics, instead of cables, brake fluid is used, and levers and eccentrics replace the cylinder-piston groups. Thanks to this approach, the friction forces in the system are significantly reduced, and this increases its service life.
Despite this, hydraulics also has disadvantages, first of all, it is difficult to repair in case of breakdown. For example, if a hydraulic line breaks down, the brakes will not work in the field. The most that is possible with the tools is to bleed the hydraulic brakes.
Hydraulic brake device
Any hydraulic brake consists of at least two cylinders with pistons, connected by a line, or, more simply, a hose that can withstand high pressure. When the cyclist presses the brake lever, the piston displaces the brake fluid from the master cylinder and transfers it to the slave cylinder located in the machine. Here, under the action of pressure, the pistons begin to move forward and press on the brake pads. And already due to the friction of the pads on the rotor (brake disc), braking occurs.
The cylinders in the brake machine are always larger than those in the brake lever. Thanks to this, in accordance with the law of hydraulics, the working brake pistons press on the pads several times more force than the cyclist does by pressing the handle. Also, this law works here due to the fact that two cylinders are installed on the brake machine, or even all 4, 2 on each side.
Symptoms of a hydraulic brake malfunction
The first symptom of a malfunctioning bicycle brake is that it starts to brake on its own. This can be explained by the fact that air has entered the brake system. This could be due to a fall of the bike, a low level of brake fluid in the reservoir, or when the hydraulic circuit opens.
Since air tends to be compressed, unlike liquids, when it enters the system, it works like a gas spring. That is, the air creates the pressure of the brake fluid, which activates the brakes.
Also, the braking system can engage in similar self-activity, in case of jamming of the working piston. And the reason for this is the ingress of water into the hydraulic system.
And, of course, the brakes urgently need to be repaired if the brake lever is not as elastic as before, or the hydraulics no longer responds to the cyclist’s command to stop the vehicle.
Troubleshooting and repair
To understand what exactly happened to the braking system of your faithful two-wheeled friend, you first need to do some experiments. The first step is to remove the wheel where the problem appeared. Next, you need to clean the brake machine well, the easiest way to do this is with a toothbrush. Well, the main goal is to remove the pads.
When access to the working pistons opens, you need to press them in with a screwdriver, and then gently press the brake lever. Both pistons should move forward. If one of them is stuck, you need to use a repair kit to fix the problem. Also, the piston system must be carefully inspected for leaks, if they are present, then the cylinder-piston group is worn out. As a rule, in this case, the pistons or special O-rings on them are changed.
In conclusion, you need to carefully inspect the entire hydraulic line. There should be no kinks, dents or other damage on it. If present, the hose must be changed.
Pumping. the final stage of repair
When the whole system has already been completely sorted out and assembled, it remains only to fill in the brake fluid and properly pump the hydraulics. The system is pumped in order to expel air bubbles from all pipes and cylinders, which prevent the hydraulics from working correctly.
There are a lot of ways how to bleed the hydraulic brakes of a bicycle. Everyone can do this at home, if they know how to handle tools a little. Someone uses syringes, and someone uses old and proven methods. Consider the simplest and most time-tested option.
To bleed the brakes, you will need the following components:
- a transparent tube, suitable in diameter to the bleed bolt;
- brake fluid or oil, depending on the specific system;
- required keys;
- jar or bottle;
- a piece of clean cloth to keep it clean.
First you need to put a transparent hose on the bleeding bolt, and lower its other end into a jar, where there should be some brake fluid. Next, you need to open the brake fluid reservoir on the handle and make sure that it is at the maximum level. If necessary, add liquid.
After that, you can start bleeding the disc brakes on your bike. Pressing the brake lever several times, you need to fix it in the pressed state and at the same time unscrew the screw for bleeding on the brake machine. In this case, you can observe how air bubbles will go out of the tube. After that, the screw must be tightened immediately, and only then you can release the handle and swing again.
This procedure must be repeated several times until the whole system works as it should. This can be felt by the stiffness of the brake lever.
Video showing the pumping procedure:
And one more way to pump the hydraulic line for the lazy:
When repairing hydraulics on a bicycle, it is unacceptable for brake fluid to get on the pads or rotor. If this happens, the elements must be cleaned immediately with fine sandpaper. Otherwise, the brake pads will not work efficiently and quickly fail.
It is also important to know that brake fluid is poisonous and can be harmful to health, so it is not recommended to get it on the skin. Eye contact should be especially avoided. In addition, it is a good solvent. easily corrodes the bulk of paints and varnishes.