This master cylinder was removed from a 80s model Suzuki GS650. I have skipped the steps of disassembling the master cylinder but they are basically the same except in reverse.
To start this project you will need to find a workspace that has plenty of room, light, and a table. Next, dig out your safety equipment. I recommend safety glasses and gloves for this project. Brake fluid is a nasty little bugger and will eat through eyeballs as fast as it will paint. The gloves will make leaving your workspace to answer the phone, etc. less challenging and it will keep the brake fluid from harming your skin. Always have shop towels in abundance and readily available. Shop towels are more durable than regular paper towels and soak up twice as much liquid.
Once your workspace is set up, check that you have all the tools you’ll need. Snap-ring pliers, needle-nose pliers, pick tools, small flathead screwdrivers, scotch-brite pad, parts cleaner, and the recommended brake fluid for your vehicle.
TOOLS

Tools: Snap-ring pliers, needle-nose pliers, pick tools, and small flathead screwdrivers.
NOTE: It is important you use your vehicle’s specified brake fluid. There are different types of brake fluids and some of them do not mix which can, and will, cause brake failure.
STEP 1

Cylinder should be clean and the walls should be smooth. All ports should be clear.
The walls of the cylinder should be smooth, i.e. not have any scratches or nicks. Scratches or nicks can tear seals or cause seals not to function properly. Scotch-brite can be used to smooth out light scratches and nicks. If the cylinder has deep ruts then the assembly will have to be replaced.
Be sure to coat the cylinder walls with the type of brake fluid you will be using before installing any parts. It’s also a good idea to lightly coat any parts going into the cylinder.
STEP 2

Install the main cap to the smallest end of the return spring. Then install the return spring, largest end first, into the cylinder.
The main cap will likely “fit” onto the small end of the return spring. In this picture, you can see a rubber notch under the main cap that fits into the spring. This notch will keep the main cap attached to the return spring while the parts are moving inside the cylinder.

The top of the main cap is visible inside the cylinder.
Gently drop the check valve down into the cylinder so that its curved side is facing the main cap. A good rule of thumb when re-installing parts is to look at their wear marks. In this case, there is a circular wear ring on the curve side of the check valve which tells us that it faces the main cap and not the piston head.

No visible wear pattern.
This is an indication that the check valve is not rubbing against something that is smaller in diameter than itself.

A visible circular wear pattern on the curved side of the check valve.
The wear pattern on this side indicates that something with a smaller diameter than the check valve is rubbing against it.
If you compare the piston head to the main cap, you will find that though they are similar in diameter, the main cap is a rubber piece held in place by the top to of the return spring. Suffice it to say, the top of the return spring is the wear pattern culprit even though it doesn’t physically touch the check valve.
STEP 3

Install the primary seal onto the piston.
This master cylinder’s primary seal has two very different sides. One side is flat and the other is cupped and tapered out farther than the flat side. Install the seal on the piston with the cupped side toward the piston head.

Piston and primary seal.
The piston head has 4 ports (only 3 are shown in the picture) the brake fluid flow will flow through, which is why the primary seal’s cupped side must face the head. The larger tapering and the cupped edge help seal fluid in so that pressure can be built.
STEP 4
It is always important to check parts for damage or wear. The majority of this master cylinder’s parts were replaced during this project due to wear. O-rings and any other rubber parts should always be thoroughly check for cracks, tears, or general wear. Brake fluid is pretty harsh so the rubber parts tend to go first. While disassembling this master cylinder, I found the piston had some scratches and nicks. These had to be remedied before I could put anything back together.

Scratches and nicks will eat up any rubber parts with which they come into contact.
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Cut a strip of scotch-brite about the same width of the piston. Then gently smooth out the scratches and nicks on the shaft.
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This piston shaft looks almost good as new!
Be careful when renewing the life of any part! The object is to fix, not change. Gently use the scotch-brite like sand paper over the scratches and nicks but don’t over do it. Changing the diameter of the piston’s shaft could be worse than it having scratches and nicks in it.
STEP 5
Now that the piston is ready to go, it’s time to install the primary seal.

Now install the secondary seal.
The secondary seal will slide onto piston with the “square” end facing the piston head. The square end has round corners to allow it to move up and down in the cylinder.

Check the secondary seal's o-ring for damage or wear. If it has signs of either, replace it.

The secondary seal should fit snugly against the primary seal.
STEP 6

Install the piston, head first, into the cylinder.
Gently drop the piston down into the cylinder. The piston won’t go far because the return spring is already in the cylinder.
STEP 7

Install the stopper plate onto the piston.
The stopper plate should drop down into the cylinder evenly and lay flat on the secondary seal.

Make sure the stopper plate is laying flat.
STEP 8

Use a small flathead screwdriver to lower the piston into the cylinder.
Lower the piston will take a good deal of pressure so make sure your assembly is secure and won’t shoot out from under you while you pushing down. When the piston is depressed completely, the stopper plate should lay evenly within the cylinder and below a visible ridge cut out of the cylinder’s wall.
STEP 9
The ridge that is now visible around the cylinder wall is there to hold the snap-ring in place. In other words, the piston will have to be held down, the snap-ring constricted, and then the snap-ring installed into the ridge. Sounds easy, right?

Snap-ring and pliers.
Stick the ends of the pliers into the loops of the snap-ring. Squeeze the pliers just enough to put pressure on the loops keeping the snap-ring on the ends of the pliers. If you don’t squeeze enough, the snap-ring will fall off, and if you squeeze too much, the snap-ring will live up to its name and possibly put out an eye!

Constrict the snap-ring by squeezing the pliers.
Once the snap-ring is inside the cylinder, more pressure can be put on it to constrict it without worry that it might pop off the pliers. However, this does not it will definitely not come off the pliers so keep your safety glasses on!

Constrict the snap-ring more to install it into the cylinder.
The snap-ring must be installed into the ridge/groove cut out of the cylinder wall just about the stopper plate. The snap-ring’s job is to hold the stopper plate in place and if the snap-ring is not in that groove, it can’t do its job.
NOTE: There are 2 grooves cut out of the cylinder wall. The lowest one is for the snap-ring. And getting the snap-ring into place will probably be the hardest part of this project so don’t freak out if it’s not just dropping into place.
STEP 10
Install the rubber boot into place over the piston. The end of the rubber boot with the protruding edge goes in first. This edge will have to be tucked into the second groove just about the snap-ring. If the rubber boot is getting tucked into the same groove as the snap-ring, the snap-ring’s in the wrong groove.
CONCLUSION
Be sure to clean up any spilled brake fluid or parts cleaner. Both of these can be harmful to plastics and painted surfaces, not to mention people in general. Dispose of your shop towels properly, especially those used to clean up brake fluid and parts cleaner.
I like to keep old parts around even after replacing them, unless they are completely useless. You never know when you might be in need of a spare in a pinch!
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