7. Front brake master cylinder: removal, examination and refitting

  1. The method of bleeding a brake system of air as described below applies equally to either the front brake or to a rear brake of the hydraulically actuated type.
  2. If the brake action becomes spongy, or if any part of the hydraulic system is disturbed it is necessary to bleed the system in order to remove all traces of air. The procedure is best carried out by two people.
  3. Check the fluid level in the reservoir and top up with new fluid of the specified type, if required. Keep the reservoir at least half full during the bleeding procedure; if the level is allowed to fall too far air will enter the system requiring that the procedure be started again from scratch. Refit the cap onto the reservoir to prevent the ingress of dust or the ejection of a spout of fluid.
  4. Remove the dust cap from the caliper bleed nipple and clean the area with a rag. Place a clean glass jar below the caliper and connect a pipe from the bleed nipple to the jar. A clear plastic tube should be used so that the air bubbles can be more easily seen. When working on the front brakes It may well prove necessary to connect both nipples at the same time and to operate them simultaneously. Pour enough clean hydraulic fluid into the jar(s) to immerse the end of the pipe: ensure that the pipe end remains submerged (to stop air returning into the system whenever the pressure is released) throughout the operation.
  5. If parts of the system have been renewed, so that it must first be filled, open the bleed nipple about one turn and pump the brake lever until fluid starts to issue from the clear tube. Tighten the bleed nipple and then continue the normal bleeding operation as described below. Keep a close check on the reservoir level whilst the system is being filled.
  6. Apply the brake as firmly as possible and hold it in this position against the fluid pressure. If the brake feels spongy it may be necessary to pump it rapidly a number of times until pressure is built up. With pressure applied, loosen the bleed nipple about half a turn. Tighten the nipple as soon as the brake lever or pedal has reached its full travel and then release. Repeat this operation until no more air bubbles are expelled with the fluid into the glass jar. When this condition is reached, the air bleeding operation should be complete. resulting in a firm feel to the brake lever or pedal. If sponginess is still evident, continue the bleeding operation; it may be that an air bubble trapped at the top of the system has yet to work down through the caliper.
  7. The description above is an outline of what can be a very time-consuming operation; great care and patience should be exercised at all times. When working on the front brakes note that if the forks are held on full left lock the master cylinder becomes the highest point in the system; this may help to clear a bubble. On the rear brake the caliper can be dismounted and hung from the frame seat tubes (with a spacer wedged between its pads and care taken not to distort or damage the brake hose and pipe) to achieve a similar result. Note also that at approximately half the lever or pedal travel air bubbles can escape from the system back into the fluid reservoir; repeated, gentle applications of the lever or pedal to this point may well release a quantity of air.
  8. In particularly stubborn cases bubbles may be released by tapping the brake pipes and hoses lightly or by topping up the reservoir and operating the brake rapidly (without splashing fluid or allowing air into the system) until the reservoir is nearly empty, to flush the system through. In some cases the only answer is to remove as much air as possible and then to leave the machine overnight (ensuring that the system is fully sealed against the entry of dirt or moisture by refitting the reservoir cap or cover and tiqhteninq the bleed nipples) so that the remaining air will build up into one bigger bubble at the top of the system.
  9. Note: A spongy brake can also be caused by fluid that has not been renewed at the required annual interval and has badly deteriorated, by detective brake hoses, by detective master cylinder or caliper seals or by a caliper piston that it sticking due to corrosion. All these points should be checked carefully if the brake remains spongy after thorough bleeding.
  10. Do not confuse excessive lever or pedal travel with a spongy feel; if the brake discs are excessively warped, for example, or if the calipers or discs are not securely fastened, the brake pads and caliper pistons will be knocked back away from the disc as the wheel rotates. This will cause a marked increase in lever or pedal travel before normal pressure is achieved; when riding the difference may not be noticed. Disc runout should be checked carefully (see Section 5 - Hydraulic brake overhaul: general), as should the security of the brake system component mountings.
  11. When all traces of air have been removed from the system, top up the reservoir and refit the diaphragm and cap or cover. Check the entire system tor leaks, and check also that the brake system in genera! is functioning efficiently before using the machine on the road.
  12. Brake fluid drained from the system will almost certainly be contaminated, either by foreign matter or more commonly by the absorption of water from the air. All hydraulic fluids are hygroscopic, that is, they are capable of drawing water from the atmosphere, and thereby degrading their specifications. In view of this, and the relative cheapness of the fluid, old fluid should always be discarded.
  13. Great care should be taken not to spill hydraulic fluid on any painted cycle parts; it is a very effective paint stripper. Also, the plastic glasses in the instrument heads, and most other plastic parts. will be damaged by contact with the fluid.

Next Section: 10 - Rear drum brake: examination and renovation - K75, K75T, K75 C

photo 9.4
9.4 Be careful not to overtighten bleed nipples - use specified torque setting, where available


Use always a new sealed bottle of fluid, because all hydraulic fluids are hygroscopic. They are capable of drawing water from the atmosphere.