Classic bike buyers are often faced with a number of dilemmas concerning their braking systems in general, and brake fluids in particular.

Generally, replacement original parts are available for most items contained within a hydraulically operated (disc or rotor type) braking system, depending on the age of the machine. This is due to the tendency for manufacturers to purchase components from specialist suppliers.

For example, AP/Lockheed supplied brakes to most of the British bike manufacturers during the 60s.

Unfortunately, one problem that is typical to braking systems is the deterioration of some of the components out of sight of the owner/operator, as in the brake fluid. Regular maintenance will keep most problems at bay, but if the service history of a machine is not known (a barn find classic, for example), there may be components within a system that may fail at any time.

With these points in mind, if the owner of a classic bike is unsure of the maintenance history, he/she must assume that the braking system is ready to be overhauled. The braking system is, fundamentally, a safety component.

A complete overhaul of a braking system would include:

• Replacing all internal components (in particular any rubber items) of both the master cylinder and calipers
• Replacement of all brake lines (either with original rubber types or stainless steel braided lines)

Once the components have been renewed, the mechanic must replace the brake fluid and bleed the system to remove any air bubbles.

Choosing the right brake fluid for a classic motorcycle should be based on the original manufacturer’s recommendations. If there is any doubt about the type or grade of fluid to be used, the classic owner is best advised to contact the brake manufacturer.

Types of Brake Fluid

There are two distinct types of brake fluid: Glycol Ether based, and Silicone based. Either base type has advantages and disadvantages, however some of the major brake system manufacturers (in particular AP) do not recommend Silicone brake fluids for motorcycle braking systems. It should be noted that that the majority of manufacturers recommend only Glycol Ether based fluids to DOT 3 specification. (Note: Harley Davidson specifies silicone based brake fluids on some of their recent machines).

DOT 3

Although the DOT 3 specification has a number of components, the main consideration for classic bike users is the boiling point/s; effectively, brake fluid has two boiling points. The first is when the fluid is new and has not absorbed any water (Glycol Ether brake fluids are hygroscopic), and the second is with approximately 3.7% water contamination.

The dry boiling point for DOT 3 brake fluid is 401 degrees Fahrenheit (205 degrees centigrade). The wet boiling point is 284 degrees Fahrenheit (140 degrees centigrade). The viscosity level is 1500 mm²/s.

DOT 4

This specification of brake fluid is mainly used for competition or high performance racing machines, as the boiling points (both wet and dry) are elevated, along with the viscosity level at 1800 mm²/s. It is a Glycol Ether /Borate Ester based fluid. The dry boiling point is 446 degrees Fahrenheit (230 centigrade) and the wet point is 311 Fahrenheit (155 degrees centigrade).

DOT 5

This silicone based brake fluid is preferred by some classic enthusiasts (primarily car collectors) as it does not damage paintwork if spilled. However, ironically, small (trace) amounts of silicone will cause ‘fisheye’ in new paintwork—it will be noticeable on the first pass of a spray gun. Although the wet and dry boiling points are elevated with silicone based brake fluids, it is not recommended for competition use due to its effect on some internal components of brake systems, and its tendency to give a soft feel at the levers.

DOT 5.1

DOT 5.1 is the current highest grade of brake fluids available. It is (as per DOT 4) a Glycol Ether /Borate Ester based fluid. The boiling points are 500 degrees Fahrenheit dry (260 degrees centigrade), and 356 degrees Fahrenheit wet (182 degrees centigrade). The viscosity limit is 900 mm²/s.

Restoration Note

With these points in mind, if the owner of a classic bike is unsure of the maintenance history, he/she must assume that the braking system is ready to be overhauled. The braking system is, fundamentally, a safety component.

With these points in mind, if the owner of a classic bike is unsure of the maintenance history, he/she must assume that the braking system is ready to be overhauled. The braking system is, fundamentally, a safety component.

During a restoration, most mechanics replace the main braking components, in particular the brake lines. If keeping the machine in its original specification is of a high priority, the mechanic should source hoses made from EPDM (Ethylene-Propylene-Diene-Materials) rubber as this type of hose is less prone to diffusion (water entering the braking system through the hose).

Alternatively, the use of stainless steel braided hoses is favored by many motorcyclists, particularly where the bike will be used in competition.

Further reading:

Replacing Broken Bleed Nipples

Classic Motorcycle Brake Upgrades