6.18. Disc Brakes

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6.18.01 Brake Bleeding

Do your brakes have a spongy feel to them? Bleeding can help. Bleeding the brake lines allows you to replace old brake fluid and eliminate any air bubbles that may be trapped in the system. Air in the brake lines is extremely dangerous because it can cause loss of pressure at the grip, rendering the brakes useless.

6.18.02 How to bleed the brakes

A WORD OF CAUTION: Any adjustment to your brake system is potentially dangerous and should be done by someone who knows what they are doing! Failure to adjust the brakes correctly could lead to loss of braking power at speed, which could be fatal.

ANOTHER WORD OF CAUTION: Brake fluid eats rubber/paint. Make sure you don't spill any on painted surfaces or get any on the outside of the rubber brake line! It can cause the brakes to slip if you get it on the rotor or brake pads...keep it off of those too!

You will need the following:

  • Bottle of DOT #3 brake fluid
  • Length of clear tubing, about 5/8" in diameter and about 3' long
  • Coffee mug or other container to catch dirty fluid
  • Strong rubber band to tie down brake lever
  • Screwdriver
  • Wrench

1. Put the bike up on the center stand.

2. Attach one end of the clear tube to the bleed screw on the caliper. Pour a little brake fluid into the coffee mug, then place the other end of the tube into the mug so that the end is completely submerged. (It helps to set the mug somewhere above the caliper for the siphoning to work)

3. Open up the master cylinder cover and fill to the top with brake fluid.

4. Slowly open up the bleed screw about 1 turn. The brake fluid will start to flow out through the tube.


5. Squeeze down on the brake lever, and watch the brake fluid flow out of the bleed screw. BE CAREFUL not to let all the brake fluid flow out of the master cylinder or you will suck air into the system and have to start over! Add brake fluid to the master cylinder as needed. Don't re-use the brake fluid that comes out into the mug as it is contaminated. Please dispose of it properly.

6. Keep pumping fluid through the system until you don't see any more bubbles coming out of the bleed screw. Squeeze and hold the lever while re-tightening the bleed screw.

7. Check the feel of the brake lever. It should feel solid. If it is still spongy, repeat the above process to get the air out.

8. Take the bike on a slow-speed test run to test and make sure the brakes are working properly.

6.18.02 Disc Brake Mods

A common performance mod for stock XS 650 disc brakes is to drill holes in the disc. This improves wet braking as it allows water and debris to escape out of the holes, keeping the pads cleaner.

Goran's Site shows an excellent illustration of how he went about drilling his discs.

6.18.04 Replacement Rotors

Front Brake Rotors

Existing rotors can be Blanchard Ground to eliminate grooves. Most larger cities should have a precision grinding shop with a Blachard Grinder. Normal cost for this is $20 to $25. According to the Clymer manual, the minimum allowable thickness is 6.5mm.

It is also possible to use rotors from an XS 1100 for the 1977 - 84 years of the XS 650. 1980 - 82 models of the XS 1100 came with slotted rotors.

80-81 SR500 G/H rotors will also fit the 1977 - 84 years of the XS 650. These rotors are 5.0mm thick.

The following is a list of rotors that I believe to fit the 77-84 XS: (Matt Q.)

81-83 XJ 550 Seca and Maxim

80-83 XJ 650 Maxim

81-83 XJ 750 Seca

81-83 XV 750 Virago

83 XV 920 K

Please note that this is based from a possibly flawed cross-reference list. Check that the outer diameter is 298mm, and the appearance is similar to your rotor. Please note that many of these are under the recommended minimum thickness of 6.5mm.

EBC sells a lightweight replacement rotor that is 5.0mm thick and can often be found for about $200.

Mike's XS also sells a lightweight replacement rotor that is 4.0mm thick. The kit from Mike's XS comes with spacers to correctly align the caliper.

(Edited by Matt Q.)

6.18.05 Dual Disc Conversion

See section 9.20. Dual disk conversion

6.18.06 Front Master Cylinder

Rebuild kits with seals and a spring are available. Be sure to clear all the holes of crud. A small wire works well. There is a 'weep hole' that is very difficult to see.

Replacement reserviors are not available - replacement of the whole assembly is recommended.

A stock US front master cylinder on 77-84 is 14mm in diameter.

Aftermarket master cylinders are available in a variety of sizes. Maguras are available in 13mm (Vintage Brake) and 16mm (Vintage BRake, Mike's XS), and Brembo is available in 11mm (Vintage Brake). 14mm or above is recommended for dual disk, and 11mm (or 13mm) is recommended for a single disk.

{Matt Q.)

6.18.07 Drilling Disks

I've drilled disks on both bikes and cars. The last disks I drilled was for my car and, at the time, drilled disks were so expensive I could buy a new drill press or two drilled disks. I went for the new drill press. (my old press was a hand cast bench press which looked good but... It's a lamp now.)

I did what I figured out in 1974 and used a sheet of paper with the disk traced on it. (be creative, try the back side of wrapping paper) After the outter diameter of the disk is on the paper the center needs to be found using a compass. Measure and draw the inner circle of the sweep area. Now say 1/4" holes are being used because there is a sale on 1/4" cobalt bits. Draw circles 1/4" apart from the bottom to the top of the swept area. Because of slight variance in hand work the holes will over lap when the project is done.

I have seen two patterns. One is draw radial lines from the center through the circles to the outside of the disk, starting with halfs, quarters, eighths. The other is to use the compass to draw an arc line over the circles starting at a point inside the disk where the half, quarter, eighth lines hit an inner circle. The holes are where the lines cross. Obviously every other hole is skipped to be hit on the next line. The latter pattern method is what I use and it looks nice.

Now you have a paper pattern. You will become so proficient at making the pattern that it will take only minutes to turn them out and experiment with different hole position. Use the tip of a colored crayon to indicate where the holes are, you don't want to get confused down the road where so much is happening. Cut out the pattern and carefully glue it to the disk. Center punch where each hole is indicated and drill.

I usually like to post some clever tip or a total disaster I did trying to be clever, this is a good one. My new drill press has a table that spins if the clamp on the arm is loosened. I found the table to spin with some precision if I loosened the clamp only enough so the table would spin but not wobble. Hey, a rotary table. I secured my disk to the table and carefully centered the disk by dragging the drill bit along the outer edge of the disk while at the same time spinning the table. The disk is centered when the edge touches the bit evenly all around the circumference, tighten the disk to the table. Drilling is almost a no-brainer once this is set up. Sure, I try to set up the disk with precision but the large number of holes makes little a variance cancel out.

I started drilling hitting each hole that was carefully marked beforehand, turning the table to easily make sure the holes were equal distance from the edge of the disk. Bump the drill press table's arm over to the next marked circle on the disk and drill those holes. The disk is drilled and it looks very cool with a unique pattern. Use a large drill bit to chamfer each hole. Just spin it by hand to break and clean the edge of the hole. Sand paper on the tip of your finger works also. I don't worry about disk balance. The hardest part is getting all the oil out of the pores of the disk metal after you are finished.

(Tom Graham 8/15/2006)

6.18.08 Drilled Disc Hole Size

There was a discussion on the list concerning hole size. Besides venting the gases the pads release, the holes can aid in cooling the disc by exposing more surface area to the air. That's if you use the proper size holes. The ideal size is 1/2 the thickness of the disc but any size up to the thickness will expose more area than the diameter of the hole removes. So, filling your disc full of large holes may look trick, but it will probably reduce it's ability to dissapate heat.

Now, half the thickness of a 650 disc (7mm) would result in rather small 3.5mm (about 1/8") holes. A good compromise is 3/16" holes. These are large enough to look OK but not big enough to cause you to lose surface area. I've drawn up a pattern using this smaller hole size .....


Bob Z.

6.18.09 Brake Fluid Specification

DOT 3, Dry Boiling Point: 205 Deg C / Wet Boiling Point: 140 Deg C.

DOT 4, Dry Boiling Point: 230 Deg C / Wet Boiling Point: 155 Deg C.

DOT 5, Dry Boiling Point: 260 Deg C / Wet Boiling Point: 180 Deg C.

DOT 5.1, Dry Boiling Point: 270 Deg C / Wet Boiling Point: 191 Deg C.

Göran Persson

New Zealand


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