DIY! How to Remove and Diagnose the Hardened Fluid in the Brake Master Caliper

How to

Disc brakes, which are standard equipment for sports bikes, can be braked by operating levers and pedals, and are released if you release the brake lever. However, depending on the condition of the calipers, the movement of the brake pads may deteriorate, and it may cause the dragging symptom. Always check the brake caliper when you want to ride after keeping the motorcycle for a long time in the garage.

Why it's important to check the remaining amount of brake pads

The KAWASAKI ZEPHYR has been stored without riding for several years when the brake lever is gripped, the front brake is left in a state of handiness. The causes of brake sticking can be attributed not only to the caliper seal problem mentioned here but also to the sticking of the slide pin of the caliper and the piston cup of the master cylinder.

As the brake lining of the drum brake becomes worn, the play of the lever and pedal increases. This is because the camshaft and brake shoe, which are parts of the interior of the drum brake, operate from the same position regardless of the remaining lining.

On the other hand, the disc brakes do not change the play of the lever and the pedal even if the brake pads are worn. The reason is that the position of the caliper pistons moving the brake pads are gradually moving out as the brake pads wear. If the brake pads wear out by 1mm, the piston is actuated from the protruding position by 1mm, and when it wears out by 5mm, it stops at the position where it protrudes by 5mm, and the lever operation pushes the brake pads further from there to pinch the rotor.

It is good that there is no trouble that the play adjustment is unnecessary, but if you do not check the wear condition of the brake pads, all the lining will wear out imperceptibly, and it may become possible to pinch the brake disc with the base plate of the base part. This base plate is made of iron, which is harder than the brake disc, so the surface of the brake disc is scraped off and the brake disc itself needs to be replaced.

Even if the brake lever play does not change, if the brake pads wear and the brake caliper piston sticks out, the level of the brake fluid in the reservoir tank of the brake master cylinder will drop. The reservoir tank has an inspection window that shows the amount of liquid, so it is important to check the brake fluid level and at the same time to check the remaining amount of brake pads with the attention to the calipers.

Click here to purchase brake pad products


  1. The play of the disc brakes does not change even if the brake pads wear.
  2. The amount of brake fluid in the reservoir tank indicates brake pads wear.
  3. Check the remaining amount of brake pads directly at the caliper.

Piston seal is the only thing that keeps the brake pads play the same

When the caliper piston is hard to come off, it is most reliable to push it out with fluid pressure before removing the brake hose. If the caliper is left alone, soak it in hot water or heat it with a heat gun. The brake fluid itself softens from the ticking state by hot water.

Piston seals and dust seals play an important role in disc brakes that cause caliper pistons to rush out according to wear on brake pads. These are O-rings with a square cross-section and are set inside the caliper while closely contacting the caliper piston.

When the brake lever is held, the piston is extruded by fluid hydraulic pressure, and at the same time, the piston seal is pulled in the direction of the thrust and deforms elastically. Then, when the brake lever is released, the deformed seal pulls back the piston with the force to return to its original position. This is called rollback, which is the most important operation of the disc brake.

The piston is pulled back by rollback, but it is not mechanically pulled back, so when the brake pads wear out and exceed the limit of the elastic deformation of the piston seal, the base point and start position of the piston change, from which the cough and pull back begin again.

The disc brakes in which does not change the gap between the brake pads and the brake disc without artificial adjustment, and as a result, the play of the brake lever and the pedal does not change and they established by the elastic deformation = rollback of the piston seal. The same applies to one-piston pin slide calipers for moped motorcycles or 6-pot calipers on the other side for supersport.


  1. There are 2 seals inside the caliper.
  2. Piston seal rollback is the most important.

Seal deterioration and alteration fluid interfere with the piston movement

If there is a compressor, it can be pulled out by high-pressure air, but when the air is blown by the air gun to the caliper, the piston pops out and is extremely dangerous. Separate the air gun from the caliper, blow air little by little, and push out the piston little by little. If the movement is bad, it is safer to connect the brake hose and push it out under hydraulic pressure. At this time, it is better to put water in the master cylinder instead of brake fluid. In that case, the water in the master cylinder should be dried after the work.

If you release the brake lever that you hold on to the disc brake, the rollback will pull the caliper piston back, but the condition of the piston seal and dust seal is important for that purpose.

Brake fluid, which is the hydraulic fluid for disc brakes, has hygroscopic property, and gradually deteriorates due to humidity and riding in the rain. The decrease in the boiling point is representative of deterioration, but it may become sediments and adhere to the area around the seal. For piston seals, it is important not to leak the fluid along with the rollback. However, due to the structure in which the piston itself protrudes with the wear of the brake pads, it can be sufficiently exposed to the outside of the seal with the fluid adhering to the outer circumference of the piston.

Therefore, when fluid is altered, when solidified and sediments with the seal groove of the seal itself or caliper, it causes troublesome symptoms such as decreased elasticity of rubber and excessive friction between the seal and the piston.

If the function of the piston seal deteriorates, the movement of the piston will also deteriorate. If you hold the brake lever, the piston is pushed out by the fluid pressure of the fluid, so the brake pads can pinch the rotor. However, if the proper rollback does not occur when the brake lever is released, the piston can not return sufficiently and the brake pads are dragged to the brake disc.

Besides, even if sediment occurs in the gap between the outer circumference of the dust seal and the seal groove on the caliper side, it may troublesome and the dust seal which does not fit in the seal groove may come off. Even if the dust seal comes off, the piston seal is inside, so the brake fluid will not leak immediately, but in this case, a caliper overhaul is required.


  1. Fluid and water combine to form sediments.
  2. Sediment can affect the seal and cause drag.

Thorough removal of seal groove sediments during overhaul

Although the dust seal does not pop out, the seal with the deteriorated brake fluid is fixed that has lost elasticity. If the brake pads are worn and left in a state where there is a lot of push-out of the piston, rust may occur in the piston itself, but fortunately, it was safe. In the case of a vintage motorcycle, some models use a dust seal with a bellows shape, but the ZEPHYR is an O-ring with a cross-section of the piston seal and dust seal.

If there is tick sediment on the tip of the caliper piston, remove it completely and replace the piston seal and dust seal with a new one. Even if you make a mistake, do not set a new seal with dirt remaining in the seal groove.

Since the brake fluid in the caliper also wraps around the backside of the seal, if the seal does not adhere to the caliper due to the dirt in the groove, it may cause new fluid leakage, and the tension may also occur in tightening the piston.

The dirt accumulated in the caliper groove is hard, and if you try to remove it roughly, it may damage the seal groove. Since the brake fluid has hygroscopic properties, it is easy to fall off by soaking hot water.

Next, when cleaning, it is important not only to clean the parts visible by the precision screwdriver or the pick-up tool but also to scrape the dirt not only at the bottom of the groove but also on the wall part so that the elasticity of the piston seal works properly. It is also effective to use a special tool to clean the caliper seal groove.

Here are the highlights of the caliper cleaning that has become dirty with sediments. Dirt accumulated in the seal groove is scraped off with a special tool.

Pick-up tools and precision screwdrivers are sometimes used, but it is important to remove not only the bottom of the seal groove but also the dirt from the walls without leaking. When cleaning the seal groove and incorporating a new seal, apply a thin layer of rubber grease that lubricates metal and rubber to reduce friction loss without twisting the seal when inserting the piston.

Besides, there is a role to protect the surface of the caliper piston against moisture, so it can be expected to have anti-rust effects.

However, since the principle that the piston begins to rush out if the brake pads wear out does not change, and if the piston with brake fluid is left unattended, sediment will eventually occur. To avoid trouble, it is effective to regularly wash the calipers with neutral detergent for lubrication and surface protection with a rubber seal assembly spray.

The dirt which had been deposited in the seal groove is completely removed, and the refresh is completed. This caliper is easy to clean because it is a one-push type, but the difficulty level is greatly increased when it is an oncoming piston. To avoid such trouble, it is most effective to periodically wash the caliper and apply rubber seal spray to remove the piston.

Apply a thin layer of rubber grease to the piston seal and the dust seal. Some service manuals instruct the application of the brake fluid, but considering that it adheres to the exposed parts of the brake caliper and pistons, it is better to use grease.


  1. Thoroughly clean the seal groove when replacing the seal.
  2. Caliper cleaning is effective before dirt accumulates.
Copied title and URL