It's in there, so it's fine! If it's running low, just refill it! We don't particularly care if it's dirty!
I believe that brake fluid is actually treated as such! All of the aforementioned factors are dead wrong. It is also important to know that brake fluid is not just a liquid, it is extremely sensitive.
Let's recap the importance of brake fluid here.
New fluid is clear and beautiful
If the reserve tank cap is removed and the inside is dirty, at the very least, you want to soak up all the dirty fluid with tissue paper and then refill with new fluid after further cleaning the inside of the tank. After the work is done, restore the reserve tank cap and slowly pump the brake lever about 5 to 6 times. This will drain out even the slightest air contamination. However, this work only replaces the dirty fluid inside the reserve tank, so you want to replace the fluid in the entire brake system as soon as possible.
Master inner cup seal incorporation is critical
Inside the master cylinder is a piston unit = inner kit, which consists of two sets of cup seals. In the past, the method of supplying parts differed depending on the motorcycle manufacturer, with some supplying pre-assembled assemblies and others supplying discrete parts. Currently, it seems that almost all manufacturers supply parts in pieces. Therefore, it is the job of the person in charge of the work to assemble the cup seals into the piston body.
While automotive cup seals are large in size and dedicated installation tools are sold, there are few installation tools available for small motorcycle cup seals. When installing, extreme care must be taken not to damage the ring-shaped cup seals. Here, we used a water supply cap purchased at a 100 yen store and a caulking cap that was no longer needed as a substitute for the special tool. Normally, the OEM SST should be used, but even a homemade tool can assemble the cup seal smoothly and correctly. By using rubber grease or metal rubber together during the work, the cup seal will slide and spread, and the assembly workability will be overwhelmingly improved. When assembling the cup seal, do not make the important orientation mistake! If the orientation is wrong, hydraulic pressure will not be applied at all = it will not work as a brake.
Rust may prevent air venting!
Since brake fluid has high moisture absorption properties, there are cases (really common) where the bleeder port for the air bleeder is clogged with rust when you notice it.
In this situation, the air bleeder cannot be opened at any time! If you try to use a syringe to bleed air or feed fluid into the system while the air bleeder is clogged, you will get a lot of air bleed and brake fluid all around you! Many SANMEC engineers must have had the experience of spraying brake fluid all over the place. When practicing fluid replacement or air bleeding, let's make sure that the internal passage of the bleeder is secured beforehand. Such confirmation alone will make the work much easier!
Let's remove dirt and rust from threaded parts and confirm that the internal passage is secured by air blowing.
Brake hose's internal fluid is also actively replaced
The brake fluid in the brake hose can be drained by connecting a syringe to the bleeder on the caliper side. After connecting the syringe, loosen the bleater bolt and pull the syringe to drain the old fluid. Fill the reserve tank on the master cylinder side with new brake fluid to maintain the fluid level as you proceed. When the syringe is filled with fluid, tighten the bleeder bolt, drain the fluid from the syringe, and repeat the same procedure. Pour out the reduced brake fluid in the reserve tank, but be careful not to forget to pour out the fluid, because air will be mixed into the brake lines. If you have a hose with a one-way valve, you can perform this operation continuously without opening and closing the bleed bolt.
POINT
Point 2 - Contamination in brake fluid is due to master cylinder inner kit seals and caliper piston seals
Point 3 - Dirt left unattended caused poor braking and master cylinder lever touch.
Moisture is the great enemy of brake fluid. Brake fluid has a DOT3 or DOT4 standard (Harley-Davidson uses silicone-based fluid, so it is labeled DOT5), but the difference in these numbers means that the boiling point of the fluid when exposed to heat is different, with 4 having a higher boiling point than 3. Since brake fluid is hygroscopic, it will absorb moisture over time with continued use, which will quickly degrade the performance of the brake fluid.
This moisture will also result in a negative effect on the inner parts that operate in the brake calipers and master cylinders. Ideally, brake fluid should be changed once a year. In particular, after a long touring trip in the rain, the brake fluid should always be replaced. When completely replacing the fluid, it is recommended to first replace the fluid in the reserve tank, and then drain the fluid in the brake lines from the air bleeder bolt on the caliper side, while replenishing the reduced amount in the reserve tank side.
In the past, many genuine brake master inner kits from motorcycle manufacturers were delivered with the brake piston itself and the cup seal assembled. However, in recent years, it seems that all the parts are often delivered in pieces. In other words, a special tool called a cup seal insert (installer), commonly known as a rocket is essential to assemble the cup seal.
It would be nice if you own the manufacturer's genuine SST, but it is not so easy. It is strictly forbidden to assemble the cup seals by pulling them with your fingertips. In such a case, we would like to recommend an insert made from waste materials. If you go to the kitchenware section of a 100 yen store, you will find a part of a product shaped like this. I once made one using an adhesive nozzle, and it was still easy to work with. The nozzle of the caulking agent was especially easy to use.
When purchasing a used car that is sold privately or as-is, it is ideal to practice a thorough inspection & maintenance of all parts before driving off the lot. The first point to inspect is the brakes. It would be best to replace the brake fluid regardless of the degree or amount of contamination. This is because fresh fluid flushes out internal dirt and sludge, which greatly affects the maintenance of brake performance. When replacing the fluid, pay attention to the condition of the air bleeder, and be sure to inspect its condition! Brake fluid is a highly hygroscopic fluid. In many cases, the internal passage of the bleeder is clogged with rust due to the high moisture absorption of brake fluid. In some cases, the bleeder may stick and not loosen, and in the worst case, it may break, which is also caused by rust due to moisture absorption. After the work, be sure to set the rubber cap after blowing air and washing out the remaining brake fluid with parts cleaner.
