I used a front master cylinder rebuild kit from Partsnmore.com
for my 1982 GL1100A Aspencade.
1. Place protective cloths underneath the master cylinder - brake fluid will eat the paint off your bike, and if it drips onto the ABS plastic of your fairing, it will cause the plastic to crack and disintegrate. I can't emphasize this enough!
2. Undo the brake line fitting banjo bolt. Be ready to catch dripping brake fluid.
3. Disconnect the brake switch wires.
4. Remove the two bolts holding the master cylinder to the handlebar.
5. Remove the master cylinder from the handlebar and to a tube that will catch the brake fluid that will be coming out of it.
6. Remove the locknut on the bottom of the brake lever assembly.
7. Remove the brake lever pivot pin.
8. Remove the reservoir cap, and empty the reservoir. Clean both liberally with brake cleaner.
8a. Now is a good time to clean the brake fluid return port in the master cylinder. It is a tiny pinhole inside a small depression - and if it gets clogged (as it will, with sludge and debris), your front brakes will drag or lock solid when they heat up. I use a small piece of stiff wire clipped from a wire brush, held with needlenose pliers to clean this out.
Note: Some reservoirs have a plastic cover with two small holes in it covering both the supply and return ports for the master cylinder. This plastic cover is part of the actual reservoir. If this is the case (you'll know because instead of seeing metal like you see in the picture above, you'll see plastic), you will need to remove the plastic reservoir from the master cylinder in order to clear the ports. The reservoir twists/pulls off, and is held in place with an O-ring. It normally takes a fair amount of force and effort to remove the reservoir. It simply pushes back on once it has been removed.
9. Remove the rubber cap from the piston end.
10. Looking into the piston area, you will see the circlip holding the piston assembly in place.
11. Depress the piston with your finger to relieve the tension, then use snap ring pliers to compress the circlip. Once it is compressed, relieve the piston and allow it to push the circlip out of the master cylinder.
12. Remove the piston from the master cylinder.
13. Inside you will see the second rubber cap. Using a small screwdriver, push the cap to one side, then pull it out.
14. Remove the spring. Clean the circlip channel inside the master cylinder of any corrosion and debris, and clean the master cylinder liberally with brake cleaner.
15. On the new spring, you will see one end that has a metal disk attached, with a hole in one end.
16. Lubricate the new rubber cap with clean brake fluid, and place it over the disk on the end of the spring.
17. Gently push the spring and the rubber cap into the master cylinder.
18. Lubricate the rubber seal on the new piston with clean brake fluid, and gently insert it into the master cylinder.
19. Place the new circlip on the piston, grab the circlip with your snap ring pliers, and push the piston into the master cylinder.
20. When the circlip reaches the channel, release the snap ring pliers and allow it to snap into the channel. Release pressure on the piston.
21. Lubricate the new rubber boot with clean brake fluid and insert it over the piston.
22. Ensure the base of the boot is pushed securely into the channel around the piston, and that the top of the boot seats correctly into the channel on the top of the piston.
23. Lubricate the brake lever pivot pin, and insert it through the frame and brake lever. Screw it into place.
24. Apply and tighten the brake lever lock nut.
25. Assemble the master cylinder back onto the handlebar and tighten it in place, making sure the angle of the brake lever is correct before tightening.
26. Reattach the brake light switch connectors.
27. Attach the brake line banjo bolt with new crush washers.
28. Fill the reservoir, bleed the brakes, and check for leaks before fitting the rubber brake line boot over the fitting.