Brake fluid needs to be an incompressible fluid with high boiling point but inexpensive. Most (DOT 3, 4 & 5.1) are glycol based, with one (DOT 5) being silicon based. Glycol based fluids are, unfortunately, hygroscopic whilst silicon based fluid are not but the latter is more compressible. The problem with water absorbed by the hygroscopic fluids is two fold:
Firstly, the water reduces the boiling point of the fluid quite significantly which can lead to gas bubbles in the system and gas is very compressible so a spongy or long brake pedal can result under heavy use. Most brake master cylinder reservoirs are not air tight so air constantly circulates through the reservoir and the hygroscopic fluid continually 'sucks' moisture from it.
Secondly, the mositure content results in accelerated corrosion of the brake pipes and calipers from the inside out. As an example, my eldest lad's first car was acquired when it was some ten years old and the brakes needed a good overhaul. When bleeding the brakes after replacing discs, pads, shoes and all rubber seals the fluid that came out looked like evil pond water - it was almost black. It obviously had not been changed in its life and the effect of corrosion was clearly evident.
Also, only use the correct specification of brake fluid as defined for the system. Brake fluid itself can be very corrosive for rubber seals etc. and the system components are designed for the fluid used. Using a different spec fluid can lead to incompatibility with system components and failure of seals.
This is a very long winded way of saying that regular changing of the brake fluid is a good idea. For normal road use I would not recommend longer periods than every three years.
Firstly, the water reduces the boiling point of the fluid quite significantly which can lead to gas bubbles in the system and gas is very compressible so a spongy or long brake pedal can result under heavy use. Most brake master cylinder reservoirs are not air tight so air constantly circulates through the reservoir and the hygroscopic fluid continually 'sucks' moisture from it.
Secondly, the mositure content results in accelerated corrosion of the brake pipes and calipers from the inside out. As an example, my eldest lad's first car was acquired when it was some ten years old and the brakes needed a good overhaul. When bleeding the brakes after replacing discs, pads, shoes and all rubber seals the fluid that came out looked like evil pond water - it was almost black. It obviously had not been changed in its life and the effect of corrosion was clearly evident.
Also, only use the correct specification of brake fluid as defined for the system. Brake fluid itself can be very corrosive for rubber seals etc. and the system components are designed for the fluid used. Using a different spec fluid can lead to incompatibility with system components and failure of seals.
This is a very long winded way of saying that regular changing of the brake fluid is a good idea. For normal road use I would not recommend longer periods than every three years.
