Just push it and walk around to see how light it is.!!Importance of mismatched brake caliper piston maintenance | News

A feature of disc brakes is that the pads open automatically when you release the brake lever. However, maintenance of the caliper pistons is essential for the smooth operation of the pads, and piston cleaning is effective when the movement of the pistons in 2-pot and opposed 4-pot calipers is unbalanced.

If the piston gets dirty, it’s harder to get back = easier to drag the pad.

When removing the caliper pistons from the car, if a pliers type piston tool or compressor air does not work, you can use tap water to push the pistons out by connecting the hose to the master cylinder. It’s a waste of brake fluid to use just to push the piston out, and with water you don’t have to worry about damaging the paint if it gets everywhere during the process. After using water, be sure to dry the master cylinder and hoses completely before restoring them.

When you release your hand from the brake lever, the piston that was pushed into the master cylinder returns to its original position by the return spring, and the brake fluid that was pushed out to the caliper side through the brake hose also returns to the reserve tank of the master cylinder. The pressure applied to the fluid in the caliper is also released, and as the fluid returns to the master cylinder side, the piston pushing the pads out returns to the caliper, and the force is removed from the pads sandwiching the rotor, thus eliminating the braking force.

Even though you may not be aware of it during normal riding, each part of the brake lever performs a variety of functions while you hold and release it. In the case of hydraulic disc brakes, thisA particularly important part of the sequence of operations is the rollback of the caliper seal.Rollback is the action of a deformed seal trying to return to its original shape. Rollback is the action of a deformed seal trying to return to its original shape, and the seal in contact with the caliper piston prevents fluid leakage from inside the caliper and returns the piston to its original position after it has moved forward. As the pads wear, the gap between the pads and the rotor increases and exceeds the seal’s deformation range, the piston automatically moves forward to maintain the proper clearance. Because of this function, disc brakes do not require the same play adjustment as mechanical drum brakes.

The distance travelled by the separated brake pads to contact the rotor is only a few millimetres, and the stroke of the caliper piston is also a few millimetres!I’m not sure what you mean by “zero”. It’s just an ideal theory, but if the wheel and brake rotor have zero shake, the gap between the pads on the rotor is 0.It shouldn’t drag even 1mm, and if that’s the case, a pin-slide caliper would have a piston of 0.A stroke of 2 mm should start braking, and conversely, releasing the finger from the lever and 0.If you go back into the caliper 2mm, the pads should leave the rotor.

In reality, however, things don’t always work out as ideal. It is important to note that a dirty piston can cause poor movement, causing the pads to drag and not leave the rotor. It is ideal to be able to brake with only a small piston stroke, but in order to execute the slight piston return accurately at all times, the piston must be able to follow the caliper seal responsively even with a small amount of rollback. To make this ideal a reality, it is important that the caliper piston has a smooth surface and is well lubricated.

When worn pad dust, grit, etc. accumulates on the surface of the piston, or when the oil that lubricates the piston and caliper seal deteriorates or dries up, the piston will not be able to return smoothly!. If the brakes are so abnormal that they stay on, everyone will notice, but if they are in contact with each other to the point where no dragging sound is made, it will be difficult to even identify them. If the brake stops immediately after a short period of idling, try pushing the caliper itself to increase the gap between the pad and rotor to make it idle again. At this time, if the friction is obviously less and the idling time is longer, it is highly likely that the pad is dragging. To be sure, apply the brakes once and let the car idle again. If the tire becomes heavy, the pad is dragging.

Utilizing a brake tool can reveal uneven piston movement.

If you set the disc brake separator and grip the lever, you will notice that one of the pistons is not coming out at all. This piston is so dirty that the correct thing to do is to pull it out instead of pushing it back into the caliper, but if you push the piston back in without thinking when replacing worn brake pads, you will end up with new pads that move unevenly.

By replacing the disc brake separator with a combination wrench, it is possible to stop the piston just before it comes out of the caliper. The thickness of the stopper varies depending on the length of the caliper and the piston, so it should be adjusted accordingly. The pistons are very dirty, but fortunately the hard chrome plating on the surface is not rusted.

Dirty caliper pistons not only reduce pad return, they also reduce the smoothness of the pad as it exits. The pistons in a single-push two-pot or opposed four-pot caliper are subjected to the same fluid pressure based on Pascal’s principle, so all pistons should have the same amount of stroke when the lever is held. However, in a dirty caliper, it is not uncommon for the pistons to fall apart when the pads are removed and the piston movement is observed. Of course, during actual braking with the pads installed and the rotor in between, the pressure applied to the pistons is greater, so even if the movement is uneven, the pads can be pushed out. HoweverIn a situation where there are no pads and the load is small, a piston with a small friction loss with the seal will try to move smoothly, while a piston with a large friction loss may not come out of the caliper for any length of timeof.

Whether you can feel it or not, when you apply the brakes with uneven pistons, the piston with easy movement pushes the pad first and hits the rotor, and the piston with big friction loss starts pushing the pad after the piston with easy movement comes out, and on the other hand, when you release the lever, the piston with small friction loss comes back quickly. When the lever is released, the piston with less friction loss will return quickly, while the piston with less movement will drag the pad forever.

If you want to observe the actual movement of the multi-piston in a simulated way, it is useful to use a disc brake separator that is used when changing brake pads. Insert the separator into the caliper with the pistons pushed back, spread the separator to the width of the pad set, and hold the lever. Then you can actually see how the easier piston reaches the separator first, and then the other piston starts to move. The way the uneven pistons move varies from caliper to caliper, but there is no doubt that dirty pistons have a noticeable effect on performance. There is no doubt about it.

Even if the pads are not worn out and the surface of the pistons are not dirty, by simply pushing the pistons out, cleaning them with a mild detergent or parts cleaner, and applying a thin layer of silicone grease, you can feel the friction loss decrease from the time you push them into the caliper. Just by restoring the brakes and pushing the bike, you will get the impression that the body of the bike is lighter. It is difficult to tell when it is time to maintain disc brakes because the lever and pedal play does not change as the brake pads wear, but if you observe the caliper and see dirt deposits on the piston surface, you should perform maintenance on the piston even if there is sufficient pad remaining.

Once the pistons are removed from the calipers, the seals are replaced.

A toothbrush and brake cleaner are not strong enough, so pull out the piston and scrub off the dirt with a rag dipped in a metal polishing chemical. Use silicone grease to lubricate the piston seals and dust seals, or apply brake fluid to the piston seals and silicone grease to the dust seals. Do not use mineral-based lubricating sprays as they may swell the seals.

The pistons were in bad shape, but the seals on the caliper side were also poor. The dust seal on the right side was swollen and sticking out from the seal groove. It was amazing that the brakes worked even in this condition. By polishing the piston and replacing the seal, the dragging of the brake was solved and the lever touch was improved.

If the piston surface is easily cleaned, the job is easy, but if the pads are worn and the piston is very dirty, it is often better to pull the piston out of the caliper and polish it. In fact, rather than using a caliper piston tool to clean the pistons by rotating them little by little, it is better to use a metal polishing chemical on the pistons themselves to remove the dirt without compromising the quality of the work. The dilemma is that you can’t polish to the base of the piston because you have to take care not to come off the dust seal of the caliper.

However, if you remove the piston from the caliper, you have to consider how to deal with the piston seal and the dust seal. According to the service manual.When the piston is removed and the caliper is disassembled, the piston seal and dust seal must be replaced with new ones. Also, there are some models that specify when to replace them.If the maintenance is only on the caliper pistons, you can avoid replacing the seals by cleaning the pistons without removing them. If the maintenance is limited to the caliper pistons, it may be cost-effective to clean the pistons without removing them, thereby avoiding seal replacement. If, on the other hand, the pistons are so dirty that crystallized brake fluid has accumulated on the outside of the dust seals, you can restore braking performance by removing the pistons and replacing the seals, as well as by carefully removing the dirt in the caliper seal grooves.

The effect of caliper piston cleaning is much bigger than you think. We recommend this work not only for riders who are particular about their suspension, but also for city riders and touring riders.