Shimano® Hydraulic Brake Service and Adjustment
This article will discuss the service and adjustment of the Shimano® hydraulic disc brake systems. For information on the disc rotor mounting and service, see Rotor Mounting and Serivce.
Disc brake systems use a caliper mounted near the dropouts of the frame or fork ends, and a rotor (disc) mounted to the hub. Hydraulic systems use sealed tubing to push brake fluid. The brake pads are housed in the caliper and are forced into the rotor.
Disc brake calipers mount to fittings on the compatible bicycle frame and fork. The common mounting standard is referred to as the International Standard. This standard uses two mounting holes spaced 51mm apart and the caliper mounting bolts are positioned perpendicular to the rotor face. A less common mounting system is the post mount. The mounting bolts of the post mount are parallel with the rotor faceand the mounting holes are spaced 74mm apart. Brake calipers designed for the post mounting system can be fitted with adapters to work with the International Standard. Brake pads for both mechanical and hydraulic systems are available in various compounds. Generally, a softer resin material will tend to squeal less. It will also offer the user more modulation, or the ability to brake lightly. However these types of pads will also wear more quickly. The harder metal or semi-metallic pads will last longer, especially in wet and muddy conditions.
Hydraulic Disc Brakes
Hydraulic brake systems use a piston at the hand lever called the "master' piston. The piston pushes brake fluid through sealed brake tubing to another set of pistons at the caliper called the "slave" pistons. The slave pistons push the pads to the rotor. Because the hydraulic fluid does not compress or flex, hydraulic systems are considered higher performance than mechanical systems.
With use over time, brake fluid will become contaminated with dirt and moisture and should be replaced. It is critical to use the correct type of fluid for the specific brake. Some manufacturers use mineral oil as the fluid, while others use an automotive brake fluid. The manufacturer will specify the type of fluid. The different types of brake fluid should never be mixed. Using the wrong fluid is likely to cause seals to fail, resulting in brake failure.Automotive fluids are DOT (Department of Transportation) approved and are generally polyglycol fluids. The D.O.T. fluids have different ratings, such as 3 or 4. Contact the manufacturer for a specific recommendation. Automotive brake fluids are caustic and toxic. Work with care to avoid fluid contact with the outside of the lever or caliper, the bike, or your skin. Use of protective gloves, such as MG-1 Mechanic Gloves is recommended.
Hydraulic systems should be inspected at all fittings and hose connections for fluid leakage and seepage. Additionally, the bike should not be stored or turned upside down, as air may enter the brake lines. If the bike has been upside down, allow it to sit several minutes before use, and test the levers by pulling with force.
The hydraulic brake lever is positioned on the handlebar similar to conventional or non-hydraulic levers. Set the angle for comfortable reach when the cyclist is in the saddle. The brake lever reach is adjusted behind the lever pivot. Tightening the screw moves the lever closer to the handlebars.
Brake manufacturers design the hydraulic cylinders at the calipers to be compatible with the cylinder at the brake lever. The diameter of the cylinder and the distance it will move (the stroke) may vary between brands. Check with the manufacturer before mixing different levers and calipers. As the brake fluid heats, it expands. Hydraulic disc systems use a reservoir system that contains a bladder to allow for the expansion of the brake fluid. Some models use an enclosed bladder in the lever, while others use an "open system."
The master piston is sealed when the lever is pulled, but is open to the reservoir when the lever in fully open. In all hydraulic systems, it is important that there is no air in the tubing or lines between the caliper and the lever piston. Air bubbles in the line will compress, causing the brake to feel "soft" when the lever is pulled with force.
For any disc system, it can be difficult to view the pad to rotor alignment. Place a white paper or white rag behind the area you are viewing. If possible, shine a flashlight on the rag for a highlighted background.
There are two different caliper mounting systems for the Shimano® hydraulic brake caliper. The caliper body may bolt directly to the rotor mounts of the frame or fork.
Alternatively, the caliper body may bolt to a bracket, and the bracket is bolted to the frame or fork. If the brake caliper is bolted directly to the mounts, it is necessary to use thin washers and shims to adjust the caliper. A washer with a post, called a "banjo washer" can be placed between the frame or fork mount and the caliper body. Install washers and secure mounting bolt. View pad to rotor alignment, then add or subtract washers as necessary.
Hydraulic systems have very close tolerances between pads and rotor. Alignment of the caliper to the rotor is critical to the performance of the brake. The procedure for Shiman® Hydraulic XTR, XT, Deore, Saint caliper alignment using an adaptor bracket is as follows:
- For bracket mounted calipers, fully loosen caliper mounting bolts. This will allow the caliper to move sideways.
- Depress the brake lever to secure pads against the rotor and maintain pressure. This will move caliper so pads are aligned to rotor.
- Inspect caliper and brake pad pistons. Push caliper left or right until pistons appear centered over rotor.
Maintain pressure on the rotor and tighten the caliper mounting bolts.
- Release lever and inspect this initial pad alignment. Ideally, the pads should clear the rotor with no rubbing. In some cases, however, a light rubbing may occur and will not generally be an issue with performance. If the wheel seems to slow when it is turned, re-adjust the pads by loosening the caliper mounting bolts to reposition the caliper.
- Fine-tune the pad alignment by fully loosening one mounting bolt while keeping the other bolt snug. This will allow you to push the caliper while pivoting off the snug bolt.
If the brake caliper is bolted directly to the mounts, it is necessary to use thin washers and shims to adjust the caliper. A washer with a post, called a "banjo washer" can be placed between the frame or fork mount and the caliper body.
Install washers and secure mounting bolt. View pad to rotor alignment, then add or subtract washers as necessary.
Brake Pad Removal and Replacement
Brake pads wear thin with use. Most manufacturers list specifications for minimum pad thickness. For Shimano® pads, replace when pad material (not including pad holder) is less then 0.9mm thick. Inspect old pads when removing. If pads are worn unevenly, it may be a sign that the caliper is misaligned to the rotor.
New disc pads may require a "burn in" period. Solvents from manufacturing are burned off from the heat of braking. Braking performance will improve after the burn in period.
As the pads wear, the pistons reposition closer to the rotor. It will be necessary to remove the rotor and push the pistons away from the center before installing new pads. The procedure for Shimano® XTR, XT, Deore, Saint pad replacement is as follows:
- Mount bike in repair stand and remove wheel(s).
- Rotate lever on handlebar until top surface of reservoir is parallel with the ground.
- Clean lever of dirt and wipe around reservoir cover. Remove reservoir cover. This will allow excess fluid to spill from reservoir.
Remove pad fixing bolt clip and unscrew pad fixing bolt.
- Remove pads by pushing them outward, away from hub axle. Notice orientation of pad return spring between pads. This spring assists pad release from rotor during braking.
- Wipe piston area clean. Use a clean rag and a mild solvent such as isopropyl alcohol to clean the piston faces and inside the caliper body.
Using a plastic lever, such as a tire lever, push both pistons into the caliper body. Push near center of piston and avoid pushing edge of piston.
- Push pistons back into caliper body.
- Place pad return spring between new pads. Pad return spring is placed between pads.
- Install pads into caliper. Orient eyehole in pads and spring to align with pad fixing bolt hole.
- Install and secure pad fixing bolt.
- Install reservoir cover and secure screws.
- Install wheel and test brake by squeezing lever with force. If lever feels soft, system will require bleeding. If pads drag or are misaligned, reset the pads.
Brake Fluid Bleeding & Replacement
Bleeding a hydraulic system is removing trapped air from the lines and calipers. Shimano brake systems use mineral oil. Never use an automotive D.O.T. brake fluid in a system requiring mineral oil. Remove the brake pads before bleeding or replacing fluid so they do not become contaminated with brake fluid. When servicing hydraulic brakes, work in clean condition. Use care to keeps hydraulic pieces, such as the bladder, clean and away from dirt.
The procedure for bleeding Shimano® XTR, XT, Deore, Saint brake fluid is as follows:
- Mount bike in repair stand. Remove wheels.
- Remove brake pads to avoid contamination by brake fluid.
Install Shimano® brake block #Y8CL18000 in place of pads. If blocks are not available, substitute a block of about 10mm wide block to prevent piston from extending. Substitute a clean 10mm hex wrench if the brake block is not available.
- Rotate bike as necessary until tubing has a continuous upward slope from the brake caliper to the reservoir. Rotate lever on handlebar until top surface of reservoir is parallel with the ground.
Attach bleed tubing to end of bleed nipple at caliper. Attach plastic bag to end of tubing to catch waste fluid .
- Clean dirt from lever and wipe around reservoir tank cover. Unthread screws at reservoir tank cap.
- Remove reservoir cap and bladder. Fill reservoir to top.
- Loosen bleed nipple, at caliper body, 1 turn.
- Operate lever repeatedly. Bubbles will appear in reservoir tank, and fluid level may drop. Keep reservoir filled with fluid. Use a non-metallic lever to tap along brake line to encourage any air trapped in the line to rise toward the reservoir.
- Keep reservoir tank filled with fluid. When oil begins to come out of bleed tube, close bleed nipple at caliper body.
- Test lever by pulling. Lever will eventually become stiff and firm when pulled. If there is no resistance to lever, open bleed screw and continue to operate handle and pump oil into the system.
When lever resistance stiffens, close bleed nipple. Hold lever closed and maintain pressure.
Using a 7mm wrench, or a small adjustable wrench, loosen bleed nipple to open system. Open and close system within one second, noticing if any of the expelled fluid contains air bubble.
- Open system briefly to expel air bubbles
- Release lever. Check reservoir tank and add fluid.
- Operate lever repeatedly. If lever feels stiff with resistance at the end of its travel, line contains no air and is fully bled.
- If lever feels soft, repeat steps 8 through 16 after re-filling reservoir tank.
- Check that reservoir tank is filled to top. Install reservoir bladder and cap. Expect some excess fluid to spill from lever. This is normal and insures no air is below bladder. Tighten cap screws.
- After bleeding, disconnect hose from bleed nipple. Wipe lever and caliper of any fluid.
Brake fluid can become dirty with use and may become contaminated with moisture. If the bike is used extensively, the fluid should be replaced once a year. The procedure for changing fluid is as follows:
- Proceed with steps 1 through 9 above.
- Operate lever to pump fluid through the hose, adding more fluid as level in reservoir tank drops. Continue to pump lever until fluid appears clear at the bleed hose. Again, check fluid and add as the level drops at the reservoir tank.
- Close nipple and proceed with the bleeding process above.
Resetting Brake Pads
If the pads seem to rub on the rotor, and re-alignment will not prevent this, the pistons may need to be reset. The procedure requires the use of a shim in the caliper for the adjustment. Shimano® supplies this shim, part #Y8CL1200, with the brake set when purchased new. Alternatively, use a hard, flat material that is the thickness of the rotor. As a alternative, the Park Tool cone wrenches, such as the Double-Ended Cone Wrench DCW 1-4, match the width of rotors and can substitute for the pad spacer.
The procedure for resetting Shimano® XTR, XT, Deore, Saint pads is as follows:
- Mount bike in repair stand. Remove wheel(s) from bike.
- Remove pad retention screw and remove pads. Use a clean rag and a mild solvent such as isopropyl alcohol to wipe clean the piston faces and inside the caliper body.
- Use a plastic lever, such as a tire lever. Push each piston back into caliper body. Notice if the piston moves after being positioned back into caliper body. If a piston moves after being reset, there may be excess fluid in the system.
- Remove cover of reservoir. Allow excess fluid to spill out of the reservoir as the pistons are pushed into the caliper body. If reservoir cap was removed, re-install cap. Wipe oil from lever.
- Install pads, pad return spring, and pad retention screw.
Install floating shim (Shimano part number Y8CL1200) or a shim of equivalent thickness.
- Squeeze lever repeatedly. Pistons will automatically center to caliper body.
- For bracket mounted caliper bodies, loosen caliper mounting bolts fully.
- Install wheel. DO NOT squeeze lever at this time.
- Inspect pad alignment to rotor. For bracket mounted caliper bodies, position caliper body to center pads over rotor. Tighten caliper mounting bolts and inspect.
- For caliper bodies mounting directly to the frame, use washers to center pads to rotor. Secure mounting bolts fully.
- Squeeze lever and inspect pad alignment. Fine tune pad alignment as necessary.