Voge brake fluid: checking, topping up and changing

Brake fluid is the only fluid on the motorcycle whose deterioration produces no warning stage. It performs, and then it does not.

Checking the level

The level is read through the sight window on the master cylinder, with the reservoir horizontal. At the front this means the handlebars turned until the reservoir sits level; at the rear it is level already.

The fluid must stand above the LOWER mark. Where it does not, the matter is urgent rather than routine.

Topping up: the 3 to 5 mm rule

The manuals give a specific target rather than an instruction to fill: the level is brought to 3 to 5 mm above the lower mark, and never past the maximum line.

The reason is mechanical. The reservoir must retain room for the fluid that returns when the pads are replaced and the caliper pistons are pushed back into their bores. A reservoir filled to the top now overflows on the bench later, and brake fluid strips paint from whatever it reaches.

A falling level normally indicates pad wear

This relationship is commonly understood in reverse. As the pads wear, the pistons stand further out of the caliper, so a greater volume of fluid remains in the caliper and the level in the reservoir falls accordingly. A slowly falling level is the normal indication of pads wearing, not of a fault.

The instruction in the manuals follows from that: when the level reaches the minimum mark, the pad thickness is checked first. Where the pads are worn, the fluid has been performing correctly and new pads restore the level without any addition. The wear limits are in brake pad wear and replacement.

Where the pads retain material and the level continues to fall, the system is losing fluid. That is a workshop matter for the same day rather than the weekend.

Changing the fluid

The maintenance plan marks brake fluid I at every service and applies a two-year limit, whichever falls first. That two-year row is the one the distance table conceals: a motorcycle covering 2,000 km a year still requires the fluid changed on time.

The reason is that brake fluid absorbs water from the atmosphere through the hoses themselves. Water lowers the boiling point; fluid that boils becomes gas, and gas compresses. A lever that travels to the bar during a long descent is that process reaching its conclusion.

The procedure given in the manuals:

1. Bring the master cylinder to horizontal, without removing it from the handlebars.
2. Open the cover and remove the diaphragm.
3. Drain the old fluid through the bleed nipple at the caliper.
4. Refill with fresh fluid of the grade printed on the cap, keeping the reservoir topped up throughout so that no air is drawn into the system.
5. Bleed until the fluid emerging runs clear and free of bubbles.
6. Refit the diaphragm and cover, and confirm the lever feel before the motorcycle is moved.

Two practices that ruin a brake system

Mixing grades. The grade printed on the cap is the grade to use. DOT 3 and DOT 4 are glycol based and are miscible; DOT 5 is silicone based and is not, and introducing it into a system designed for glycol destroys the seals.

An opened bottle kept in the workshop. Fluid absorbs water from the air inside the bottle exactly as it does through the hoses. A bottle opened a year ago is not fresh fluid. The small container, bought for the job, is the correct purchase.

Related pages

The discs the fluid ultimately acts on, with their wear limits, are in brake discs and calipers, and the service at which each brake item falls due in what Voge asks for at each service.