Shimano use mineral oil in their hydraulic brake systems. This is significantly less toxic than DOT fluid, so is not as damaging to your skin and your bike’s paint but it’s still a wise move to wear gloves.
Make sure that you replace the fluid with the same mineral oil; filling it with DOT fluid will damage the seals irreparably and you’ll end up with a very big repair bill. Shimano sell a dedicated bleed kit for their brakes which includes a single-use bottle of oil containing enough for one (careful) bleed. Conveniently, this means that there’s no leftover oil to sit around absorbing moisture from the atmosphere. Mineral oil changes colour as it goes ’off’, so if you haven’t bled the brake for some months and are starting to suffer poor braking, remove the top cap of the brake lever and check the fluid – if it’s lost its pink tint then you need to replace the oil.
You’ll be pouring the fresh oil straight into the master cylinder at the brake lever, which can be messy. Wrap a rag or paper around the lever before you begin as a pre-emptive mopping up exercise, and keep a wary eye out for oil running down the hose towards the front brake.
Step 1: Start by removing the wheel and the brake pads from the brake you want to bleed, to prevent them becoming contaminated. Loosen the brake lever’s clamp bolt, then swivel the lever around the bar so that the top cover is horizontal. Tighten the clamp bolts to keep the lever in place, then remove the top cover of the reservoir.
Step 2: Once the top cap is off, you’ll see a rubber diaphragm. Remove it gently and place the top cover and diaphragm in a safe place, on a clean rag as they will be oily. Top up the reservoir with fresh oil.
Step 3: Push the pistons gently back into the calliper so that they’re flush with the calliper body, then wedge them into place with either a fold of sturdy cardboard or the yellow plastic block supplied with all after-market Shimano brakes.
Step 4: As long as the brake lever is above the calliper and the hose is running in a vertical direction with no sharp bends, you shouldn’t need to remove the calliper from the frame or fork mount. Fit the plastic hose supplied with the bleed kit to the bleed nipple, and then attach your chosen receptacle to the end of the hose. A plastic bag is supplied with Shimano’s bleed kit but a small plastic bottle with a hole in the lid is a more practical receptacle to catch the oil.
Step 5: Using a crescent wrench, open the bleed nipple and pump the brake lever. You may see air bubbles appear in the reservoir and the oil level may drop. Keep topping up the reservoir and pumping the lever, until oil appears in the bleed hose at the calliper. Close the bleed nipple tightly, then pump the lever – it should firm up quickly. If not, then repeat the process, tapping the hose gently to encourage any air bubbles lodged in the hose to travel up to the lever.
Step 6: Once the system is full of oil and free from air, close the bleed nipple, pull the lever back as far as it will go and quickly open and then close the bleed nipple. The bleed hose will fill with oil and there should be no bubbles in it – if there are, you will need to continue topping up the reservoir. Once the bleed is complete and the brake is firm, top up the reservoir, replace the diaphragm and top cap, return the lever to its usual position on the bar. Remove the bleed hose and replace the bleed nipple cover, clean up the calliper. Replace the pads and wheel before testing brake operation.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES