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The brakes are the safety-relevant component on the racer. If the stoppers fail before an obstacle or a tight bend on a fast descent, a crash is inevitable. Disc brakes are the state of the art in road bike technology, offering better deceleration values and better modulation in every riding situation compared to classic rim stoppers with cable. However, as with any highly developed technology, their optimum performance requires regular maintenance and care. Typical system problems are brake fluid with an increased water content or air in the brake line, for example due to a defective seal. Signs that there may be air in the system are a spongy feeling when pulling the brake lever and a wandering pressure point, which initially comes late, but increasingly earlier when "pumping" with the lever. The lever can initially be pulled further than usual without the braking force increasing accordingly. This is because the air in the line is compressed instead of transferring the pressure directly to the brake pistons via the brake fluid. Another sign may be a delayed or uneven pressure point, which makes it difficult to precisely meter the braking force. It is therefore necessary to regularly replace the mineral oil or DOT brake fluid in the brake system, but at the latest as soon as the spongy feeling described above becomes noticeable when the brakes are applied.
DOT or mineral oil?
Mineral oil (Shimano, Campagnolo) is less hygroscopic (water-attracting), while DOT brake fluid attracts water more and is also corrosive. When handling DOT fluid, safety goggles and gloves are mandatory to protect the skin and eyes.
Mineral oil and DOT brake fluids are used as the medium. Both are hygroscopic, which means that they attract water from the ambient air over time. This water absorption can significantly impair braking performance. With DOT brake fluid, the water lowers the boiling point, which can lead to the formation of vapour bubbles during intensive use and then to complete brake failure. Although mineral oil is less hygroscopic, impurities and moisture can also reduce the efficiency of the brake system. Regular filling and bleeding of the brakes ensures that the brake fluid is free of air bubbles and dirt. It also extends the service life of the brake components, as clean brake fluid is less corrosive and protects the seals and pistons. In the instructions below, we explain step by step how to do it using Sram as an example and what fundamental differences there are between Campagnolo and Shimano. For those who are confident enough to tackle this task, we also recommend the model-specific maintenance plans of the brake manufacturers with further detailed information such as torque specifications and spanner sizes.
In my experience, patience is required when filling and bleeding, as air bubbles tend to hide in the lever or brake calliper. - Matthias Fischer, Mechanic
1. relax first
For models with grip width adjustment, the largest grip width must be set before filling and bleeding so that the maximum fluid volume in the system can be utilised. On Sram, the adjustment screw (3 mm hexagon socket) is located at the front at the level of the joint under a small cover.
The cutlery
The following protective equipment is recommended when handling brake fluid Gloves and Safety goggles. Depending on the manufacturer, various Torx and/or hexagon socket spanners and needle-nose pliers may be required; excess brake fluid can be removed with a cleaning cloth and isopropyl alcohol.
Bleeding sets
Sram
The Bleeding Kit Pro from Sram is very comprehensive and contains two syringes of DOT 5.1 brake fluid, rubber seals in various diameters, hose connectors, various spanners and so-called bleeding blocks, which serve as spacers between the brake pistons when the wheel and brake pads are removed. Price: 116 Euro >> available here.
Campagnolo
The standard Campagnolo set only contains two syringes plus two screw adapters, one of which is equipped with a hose extension. The price for this is just under 43 euros >> available here.
Shimano
Instead of a second syringe, a screw-on funnel with a stopper serves as a fluid reservoir. Three different plastic wedges as placeholders complete the Shimano service kit. Price: 52 Euro >> available here.
2. draw up the syringe
After preparing the necessary tools, the syringe with the appropriate threaded connection (black screw thread) is filled three quarters full with brake fluid. It is almost inevitable that some air will be sucked in. To ensure bubble-free filling, turn the syringe with the opening facing upwards and press the air out of the cylinder and hose. Gently push the plunger upwards so that the sluggish air bubbles have time to escape. Holding the syringe against the light makes this process easier to recognise.
3. blocking aid
Removing the wheels and brake pads is necessary and makes it easier to handle the syringes. The inserted bleeding block ensures that the brake pistons are pressed to the end stop. This ensures that the old brake fluid is completely displaced when the fresh fluid is added.
4. collect old liquid
If the brake pistons are blocked, the second, empty syringe with the red wing nut (bleeding edge adapter >> available here) is connected to the bleed connection in the brake calliper.
5. open access
You should be well prepared before opening the Torx screw at the top of the brake lever: The bleed screw must be at the highest point so that as little brake fluid as possible can escape. A cleaning rag bandage around the brake lever absorbs any escaping brake fluid before it can spread over the wheel and floor. When removing the screw, pay attention to the small O-ring seal so that it does not get lost.
6. bubble-free
If you only want to bleed the brakes, save yourself step 4 and open the hose clamp after screwing in the adapter. Air escapes in rising bubbles. Pulling on the piston creates negative pressure, which accelerates the ascent; light tapping releases bubbles stuck in the lever or brake calliper. As soon as no more bubbles rise, apply a little pressure to the piston, then the adapter can be loosened and the access closed with the screw and O-ring.
7. interplay
By pressing on the piston of the upper syringe, the brake system is now filled with the fresh brake fluid and at the same time the old fluid is displaced downwards into the empty syringe - recognisable by the slight cloudiness. As soon as the fresh and clear brake fluid becomes visible at the bottom and no more air bubbles escape, the process is complete. The bleed nozzle is now closed and the lower adapter with wing nut can be unscrewed again.
8. with negative pressure
Tip: A slight pull on the syringe plunger prevents brake fluid from spilling unnecessarily when disconnecting the adapter.
9. build up pressure
Before the final function check, check again that there are no more air bubbles, supported by lightly actuating the brake lever and pulling on the syringe piston. If a clear pressure point can be felt, disconnect the filling hose and unscrew the adapter from the brake lever. The procedure for the rear brake is identical, although a little more brake fluid is required due to the longer line from the lever to the caliper.
10. clean & pure
Oil and brake fluid residues must be removed after the service so that they do not attack rubber parts, handlebar tape or paintwork. A cloth or paper towel is sufficient for rough cleaning. Isopropyl cleaner, which is also used to clean sound or video heads, is more thorough. This inexpensive cleaner is gentle on surfaces and cleans without leaving streaks. Brake cleaner also cleans well, but leaves painted surfaces looking a little dull afterwards. The old brake fluid must be disposed of properly. Private individuals can find special containers for this purpose at the nearest recycling centre.
This is different with Campa and Shimano
Open at the top: In contrast to Sram, the Campagnolo syringe has a hole in the cylinder; the Shimano screw-in funnel is open at the top and can be closed with a small plug. This means different handling when bleeding, as air in the line can escape from the system more quickly upwards without back pressure. At the same time, more care is required as the mineral oil can be spilt more easily.
With pressure from below: With Campa and Shimano, the fresh oil is pressed from the brake calliper, i.e. from below, into the line up to the pump in the handle. To do this, the racers must be tilted slightly downwards, in the case of Shimano clearly visible by the horizontal fill level in the funnel. In addition to the bleed socket on the brake calliper, to which the adapter is connected, a second bleed screw must also be opened before filling.
Matthias Borchers
Editor
