Disc Brakes Fluids & Their Types

Let us talk about disc brake fluids and some of their details. There are few other types also available apart from the most widely used DOT 3 & 4.

When we talk about high performance vehicles we also think about brakes like Mechanical linked Brakes, Hydraulic Brakes and Air Brakes. This article is completely related to hydraulic disc brake systems which is used in Indian motorcycles. Extensive usage of Disc Brakes in Motorcycle, neglected brake service due to cost of spares, lack of knowledge about servicing and so on.

Disc brake fluids
Pic source – How Stuff Works

The image describes how a disc brake works. You can read about Disc Brakes in detail in our following series..

Lets think about the material we use and get for that. Caliper & master cylinder we get from company is made of aluminum and we use DOT 3 or 4 fluid for activation and so company recommends that too.

DOT 3 and 4 only grades for hydraulic brakes? No, A total of four grades are available DOT 3, 4, 5 and 5.1.

  • DOT 3 or 4 (Glycol Based): Maximum we use these fluids in our motorcycle. Well because we know only about those two grades and we get only these two grades.
  • DOT 5 (Silicone Based): Not widely available. Not suitable for ABS
  • DOT 5.1 (Glycol Based): Not widely available but this fluid has HIGH BOILING POINT and can mix with DOT 3 or 4. So this is the best fluid for our machines.

Servicing of disc brakes?

Well, DOT 3/4 fluids are very high hygroscopic (hygroscopic means they absorb water easily and hold it in suspension). When we apply disc brake at that time it boils at very low temp due to that it expands the internal air which is in heat expansion chamber (this chamber is in the reservoir below the master cylinder cap, and above the max level of fluid).

When air get heats up due to the internal temp it also cools down. As we know when air heats up it becomes steam but after cooling it becomes water. As I told you before about hygroscopic characteristic of fluid means it can mix with water easily. Generally our mechanic says that water is entering from the hole which is given for breathing purpose. But NO, that is not the only reason. Heat, cold & moisture is the reason behind all the mess.

The material which companies generally use for caliper and master cylinder is aluminum or sort of aluminum alloy. Aluminum never gets rusted but it does get affected by water so that shows deposition of oxidation on the surface.

In the master cylinder when fluid mixes with water, it affects the master cylinder body and starts the oxidation reaction. The oxidation will never mix with fluid but floats on the surface or sticks to cylinder wall or it goes to the main track till caliper. And here you feel bad brakes. First brake fluid becomes black due to temperature & operation; secondly oxidation clogs the track. Result: Bad and hard brake lever or no brake at all.

Disc brake fluids

So first of all service the brake system thoroughly and change the brake fluid at once every year. The level given on the master cylinder is not to refill the fluid but that indicates the brakes pads wear and tear. So if the fluid is less, then change the brake pads as soon as possible.

Brake fluid generally does not decrease by itself unless there is any accident or leakage. But at normal usage pads start wearing. You can buy your preferred brake fluid from this long list available on Amazon.

Always check and get cleaned the disc unit from professional and skilled mechanic. Do not try to attempt this at home if you are unknown to any mechanical parts. This article is for awareness only.

– Chinmay Dangre