Motorcycle disc brake servicing typically includes pad replacement and brake fluid flushing or bleeding. Brake fluid flushing is often synonymously referred to as brake bleeding, although flushing and bleeding are different operations. Bleeding refers to removing air from a brake system; flushing refers to removing old brake fluid and replacing it with new brake fluid. This article covers replacing the brake pads and bleeding or flushing the brake fluid. Our demonstration mule is a 1982 1000 SP Moto Guzzi. This Moto Guzzi (and other Guzzis of the era) use the two-piston Brembo F08 caliper.
The brake fluid flushing operation is somewhat unusual due to the 1000 SP Guzzi’s linked brake system. The front brake lever operates the right front disc brake caliper only (the Guzzi has twin discs on the front wheel); the rear brake pedal operates the front left disc brake and rear disc brake calipers simultaneously. This article focuses on the calipers operated by the rear brake pedal; the maintenance operations for the right front disc brake are similar and simpler (but not included here).
Service intervals
Cycle Garden’s Moe Moore advises inspecting your brakes every 3,000 to 5,000 miles. Brake pads need to be replaced when the pads are worn excessively on either side, when the pads are contaminated with lubricant, or when the pad thickness is less than 1.5mm (0.06″ or about 1/16″). Brake pads should be replaced before the pads wear down to their mounting material. If this is allowed to occur, the brake pad mounting material will dig into and score the brake rotors. Brake rotors are much more expensive to replace, and much harder to find for vintage motorcycles.
The 1982 Moto Guzzi 1000 SP uses two different systems for brake fluid level monitoring. The front master cylinder on the handlebars uses a window that allows assessing fluid level visually without opening the front master cylinder. The left front caliper/rear caliper master cylinder (located under a body panel) is not translucent; it instead uses a float-actuated level switch that illuminates a red warning indicator on the motorcycle’s dash display if the fluid drops below an acceptable level. As the brake pads wear, the fluid level in the associated master cylinder will drop. A low fluid level in the master cylinder can indicate pad wear. It can also indicate a brake fluid leak.
Brake fluid should be flushed every one to two years, if the brake fluid is discolored, if the motorcycle is operated in a humid environment (brake fluid is hygroscopic), or if the motorcycle is ridden aggressively or raced. If the brakes feel spongy or if the hydraulic circuit that actuates the brakes is opened, the brakes should be bled.
Required tools and materials
- Brake pads
- DOT4 brake fluid
- Brake pad lubricant
- Brake cleaner
- A 17mm socket and drive
- An 8mm wrench
- A 1/8-inch punch
- A hammer
- Nitrile gloves
- Shop rags
- A helper to assist in the brake bleeding or flushing process
Safety considerations
Warnings: As is the case for any motorcycle maintenance operation, secure the motorcycle such that it will not tip over, and wear nitrile or other impervious material gloves while working with brake fluid. If any brake fluid contacts your skin, wash the affected areas promptly and thoroughly.
Cautions: Brake fluid can damage painted finishes, so when flushing or bleeding the brakes, protect any painted areas near the master cylinder and immediately wipe up any brake fluid that contacts painted surfaces. When bleeding or flushing the brakes, some people attach a clear hose to the brake bleed fitting to route escaping brake fluid to a catch bottle (not illustrated here; you may wish to do so). This allows you to observe dirt or air flushed out should there be any. When the brake calipers are detached from the motorcycle, do not allow them to hang by the hydraulic hoses as doing so will weaken and damage the hoses. Support the caliper by hand or by re-attaching it to its mounting points.
Terminology
Relevant brake caliper components are shown at the top of this page.
- A. Caliper. There are three calipers on the 1000 SP. The rear caliper is different from the two front calipers, as it has two brake fluid bleed fittings rather than the single brake fluid bleed fitting on the front calipers.
- B. Bleed fitting. The bleed fitting can be opened to allow brake fluid to escape during the brake fluid flush and bleed processes.
- C. Shims. These are simple washers. They are used to center the caliper (left to right) over the rotor. The calipers also have scribe lines to assist in centering the caliper over the rotor.
- D. Mounting bolts. There are two, one at the top of the caliper and one at the bottom.
- E. Brake pads. There are two (one on either side of the disc). In this article, we use DP Brakes Part No. DP600 sintered brake pads.
- F. Through pins (also known as locating pins). There are two for each caliper. These pass through both sides of the caliper and both brake pads. They locate the brake pads with respect to the caliper and the rotor.
- G. Anti-rattle pin. There is one for each caliper. This transmits downward pressure from the clip to the brake pads and prevents brake pad rattle.
- H. Clip. There is one for each caliper. It is captured by the front and rear through pins and the anti-rattle pin, and provides downward pressure on the brake pads to prevent rattling.
1. Secure the motorcycle
Whenever working on the motorcycle, make sure it is secure so that it does not tip over and injure you or damage the motorcycle.
2. Unbolt the caliper
Without detaching the hydraulic line to the caliper, remove the front caliper by removing the two 17mm bolts attaching the caliper to the front fork. Do not allow the caliper to hang by the brake hydraulic line.
3. Remove caliper through pins, clip, anti-rattle pin, and brake pads
While supporting the front caliper by hand, remove the caliper through pins, the clip, and the anti-rattle pin. Set the parts aside.
After the brake pads have been removed, do not actuate the brake lever. Doing so will extend the caliper pistons, which will have to be pushed in again so that the new brake pads can be installed.
4. Push pistons back into caliper
Because the brake pads removed are worn, the pistons will protrude into the area occupied by the brake pads. These pistons need to be pushed back into the caliper to allow installation of the new brake pads.
5. Remove new brake pads from packaging
As mentioned at the start of this article, we installed new DP Brakes DP600 brake pads. Three brake pad pairs are required for the Moto Guzzi 1000 SP (one pair for each caliper). Note that each brake pad has two sides. The side with the thicker brake pad material faces the brake rotor.
6. Apply brake pad caliper lubricant to brake pads
Disc brake caliper lubricant helps to eliminate brake noise. It should be applied sparingly only to the brake pad area that faces the caliper piston. Do not apply excessive caliper lubricant (it may migrate to the brake pad surfaces and degrade braking). Do not apply caliper lubricant to the brake pad material that contacts the rotor.
7. Reassemble caliper
This step involves reassembling the brake caliper by installing the caliper pads, the through pins, the anti-rattle pin, and the clip.
8. Bolt brake caliper to fork
After installing the brake pads, caliper through pins, anti-rattle pin, and clip into the caliper, it should be remounted to the fork lower.
Repeat the above brake pad replacement process on the other two calipers (the right front and rear calipers). These are similar to the left front caliper, with the exception of the rear caliper having two bleed fittings.
9. Brake fluid bleeding or flushing
Brake fluid bleeding is an operation that removes air from the brake hydraulic circuit. Bleeding is required when the brakes feel spongy (usually because air has been introduced into the brake fluid), or if the hydraulic circuit has been opened.
Brake fluid flushing is an operation that removes a brake hydraulic circuit’s old brake fluid and replaces it with fresh brake fluid. Flushing is required based on a specified time interval, or if the brake fluid is discolored, or if the brakes have been subjected to excessive heat cycling (as would occur in racing). Brake fluid discoloration can be due to moisture absorption (brake fluid is hygroscopic) or exposure to excessive heat (as would occur in racing or extreme use). Most motorcycle owners’ manuals recommend replacing the brake fluid every 1 to 2 years. In high humidity areas brake fluid flushing might be necessary every 6 months.
Brake fluid bleeding or flushing require similar steps. When bleeding the brakes, we open the caliper bleed fitting, pump the brake lever or pedal, close the fitting, release the brake lever or pedal, add brake fluid to the master cylinder, and repeat the process until the emerging brake fluid is free of bubbles. When flushing the brake fluid, we do the same except we repeat the above process until new brake fluid emerges from the bleed fitting. Used brake fluid is opaque and darker in color than new brake fluid, so we watch for fresh brake fluid to squirt out of the bleed fitting.
With a linked brake system like our Moto Guzzi 1000 SP has, the rule is that the caliper farthest from the master cylinder is bled or flushed first, and then next closest caliper to the master cylinder is bled or flushed next. On our 1000 SP, the master cylinder for the left front and rear brake calipers is located behind a panel beneath the seat. Because the left front caliper is farthest from this master cylinder, it will be bled or flushed first. The 1000 SP’s rear caliper has two bleed fittings, and for the same reason, the rear caliper outboard bleed fitting will be bled or flushed next, and finally, the rear caliper’s inboard bleed fitting will be bled or flushed.
Bleeding or flushing the right front brake caliper is not covered here, but the procedure for bleeding or flushing is similar to the left front caliper, except for its master cylinder location on the right handlebar and the use of the handlebar brake lever for pumping brake fluid through the system.
Our special thanks to Moe Moore and Steve Howlett at Cycle Garden in Indio, California, for walking us through the procedures covered in this article, and to Larry Mills at DP Brakes for providing their excellent brake pads.
Rotors, brake pads, and brake fluids
The vast majority of street motorcycles use either stainless steel or cast iron rotors (racing motorcycles may use carbon fiber or carbon-ceramic rotors, but those are not discussed here other than to mention their existence). Stainless steel rotors are used on nearly all modern street motorcycles; their principal advantage is that they don’t rust and therefore, they look better. Cast iron rotors are original equipment on older motorcycles. Cast iron rotor advantages include superior braking force and reduced susceptibility to brake fade; the disadvantage is that cast iron rusts easily.
There are four brake pad compositions: sintered, semi-sintered, ceramic, and organic. Our recommendation is to replace your brake pads with the type originally installed on the motorcycle, but sometimes riders prefer an alternative brake pad composition. The different brake pad materials and their advantages and disadvantages are as follows.
Sintered brake pads: Sintered pads are composed of metallic particles (usually a copper alloy) compressed under high heat and pressure, with the resulting sintered material then fused to a backing plate. Sintered pads offer good stopping power (even when wet) and long life. Sintered pads are sometimes noisy, may wear rotors more quickly, and they can transfer high heat to the brake fluid.
Semi-sintered brake pads: Semi-sintered brake pads combine organic pad material with a reduced metallic particle content. They are a compromise between sintered brake pads and organic brake pads, providing both a strong bite and an input-proportionate brake feel while abrading rotors less than sintered pads. They do not stand up as well as sintered pads to repeated high temperatures.
Ceramic brake pads: Ceramic brake pads are intended for racing applications. They are similar to sintered brake pads, except that ceramic materials are used instead of the sintered pads’ metallic materials. Ceramic brake pad material is similarly fused together under high heat and pressure and then bonded to a metal backing plate. Ceramic brake pads take longer to warm up, and they are more expensive than sintered brake pads. They are not intended for use with cast iron rotors, as they will wear the rotors excessively.
Organic brake pads: Organic brake pads (sometimes referred to as resin pads) are composed of rubber, silica, and Kevlar particles bonded into a resin base. They are quieter than sintered brake pads, they work well in low temperatures, and they transfer less heat to the brake fluid. Organic pads provide less braking force, they are more susceptible to fade, they do not perform as well as the other pad materials when wet, they are prone to glazing, and they do not last as long as sintered or ceramic brake pads.
The primary difference between DOT 3 and DOT 4 brake fluids is that DOT 4 has a higher boiling point. This makes DOT 4 brake fluid better for higher performance applications. It reduces the propensity for brake fade under high performance demands. DOT 3 fluid costs less, and it absorbs moisture more slowly than DOT 4 fluid. DOT 3 brake fluid should never be used in a system that specifies DOT 4 fluid.
The 1982 Moto Guzzi 1000 SP featured here came from the factory with cast iron rotors and sintered brake pads. Some owners replace the cast iron rotors with stainless steel versions and the sintered brake pads with organic brake pads to reduce the propensity of the rotor to rust and to eliminate “grabbiness” and brake noise. For this article, we used DP Brakes DP600 sintered brake pads, and we stayed with the motorcycle’s original cast iron rotors. The Moto Guzzi 1000 SP specifies the use of DOT 4 brake fluid.
