Plus caliper piston rub out. Easy maintenance of the master cylinder to improve the lever touch!

Regularly checking the brake pad level and rubbing out the caliper pistons will keep your disc brakes going strong. If you still feel that "the grip on the brake lever is not good enough?" then pay attention to the master cylinder piston. A simple maintenance to eliminate dirt and friction loss caused by aging and poor lubrication will smooth out the reluctant lever touch and enable smooth braking.

Lever touch critical for subtle brake control

Disc brakes exert braking force when fluid pressure is applied from the master cylinder to the calipers by operating the brake lever or brake pedal. While it is the frictional force between the brake disc and brake pads that ultimately determines braking effectiveness, there are other factors that affect the feeling during braking. Rubbing of the brake caliper pistons is a prime example. By cleaning the caliper pistons of worn brake pad dust, sand, and dust, and applying a chemical like MR20 that is suitable for lubricating the metal (piston) and rubber (dust seal and piston seal), the pistons will move smoothly. Smoother piston movement does not necessarily mean improved braking power when the lever or pedal is held or pressed at 100%. This is because the maximum output relative to the input is determined by Pascal's principle based on the piston area of the master cylinder and caliper. The improvement in feeling can be experienced when the caliper is rubbed out, because of the good start-up when the brakes are applied and the good release of the pads when the brakes are released. The master cylinder is another part that affects brake feel along with the calipers. Considering that it is an input part, its changes may be felt more subtly than those of the caliper.

The first step is to clean and lubricate the brake lever pivot bolt

Here we will focus on the front brake master cylinder, but the first point to maintain is the brake lever pivot. Although too obvious to be overlooked, the pivot bolt that supports the brake lever and the sliding part of the lever must be lubricated with grease. Poor lubrication due to lack of grease and dirt can cause increased friction loss during lever operation as well as wear on the bolt and lever. If the clearance is widened due to wear of the sliding parts, the brake lever will rattle and the touch will deteriorate, which may lead to wear of the lever and master cylinder. Some people try to tighten the pivot bolt too hard in order to reduce the rattle between the master cylinder and the lever, but most pivot bolts have a stepped structure that does not narrow the width of the lever holder portion of the master cylinder. Also, since the tightening torque is set low, care must be taken to avoid damaging the female threads on the master cylinder side if the bolt is tightened with excessive force. If the rattling becomes too large, the brake lever and master cylinder will eventually need to be replaced.

When the "creaking" feeling in the lever does not disappear, suspect the master cylinder piston.

If, after greasing and restoring the brake lever pivot bolt, you still feel a "creaking" sensation when you grip the lever, as if something is catching or rubbing, maintenance of the master cylinder piston may help the symptoms. The master piston seems to stroke in a straight line with respect to the master cylinder when the brake lever is squeezed, but strictly speaking, a slight diagonal prying motion is added. This is evident from the fact that the contact marks left by the brake lever on the master piston are not dots but lines. In other words, as the amount of brake lever grip increases (i.e., the amount of piston stroke increases), the contact area between the lever and the piston moves and pushes into the piston. The piston length is long enough to prevent the piston from tilting as much as possible during the stroke, and the instruction is to lubricate the sliding parts during assembly. However, if lubrication is reduced or lost over time, the piston and cylinder may rub against each other, increasing friction loss. Although primary and secondary rubber piston cups are incorporated at the contact points between the two, metal-to-metal contact can occur if the pistons are pried apart by lever operation. In addition, some models specify brake fluid application to the sliding parts when assembling the master piston. In this case, the fluid that adheres to the piston must be carefully monitored for deterioration over time. Generally, brake fluid is specified to be changed every two years, and the fluid inside the caliper is replaced from the reservoir tank of the master cylinder. Rarely is the piston removed from the master cylinder to be cleaned at this time, and the fluid applied to the piston may gradually deteriorate, causing deposits to form. These are solids that have oxidized from the brake fluid absorbing moisture and interfere with the movement of the master piston, causing a creaking sensation. The best way to remove the solids is to overhaul the master cylinder. If the piston is pulled out of the master cylinder and rinsed with water (hot water), the ticking deposit can be removed and the piston will stroke smoothly and the operation of the brake lever will be improved. If you cannot take that much trouble, it is also effective to clean the master piston with parts cleaner or MR20 while pushing it in with a flathead screwdriver or flat punch. It is also effective to temporarily remove the brake pads so that the pistons can be pushed in deeper. If you stroke the piston several times while spraying parts cleaner or the like, you should be able to restore the smoothness when you start to grip the lever.

In addition to cleaning and lubricating the master piston and cylinder, try changing the position of the piston hit

In addition to cleaning the master piston, changing the contact position between the piston and master cylinder can also improve lever touch. As explained earlier, in some vehicle models, the master piston may stroke while being subjected to diagonal prying forces. If the contact area with the brake lever is concentrated in a particular location, the piston end may become unevenly worn, further deteriorating the straight stroke. In such a case, it can be corrected by overhauling the master cylinder, removing the piston, and using an oil stone to even out the depressions on the contact surface with the lever. On the other hand, if you are not able to do that much, simply rotating the piston and changing its contact position with the lever can also be effective. This will simultaneously change the contact position of the cylinder and piston, which is expected to be a favorable factor. Although the master cylinder piston does not get contaminated during driving like the brake caliper pistons do, deteriorated brake fluid can cause increased friction loss. If the lever touch is still uncomfortable after maintenance of the brake lever pivot bolt, it is a good idea to check the condition of the master piston.

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