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Yes, brake fluid should be changed on a regular basis.
But, what constitutes 'regular'?
Well it should definately be changed if the moisture content is 3% or more. This can be determined in a number of ways, but the easiest is to buy a brake fluid tester.
Since most people do not have a brake fluid tester, manufacturers specify a change interval, usually in the range of 2 to 4 years.
Some think that just changing the fluid in the reservoir is ok because apparently the fluid is only exposed to the air (and hence moisture) at the reservoir.
This is wrong. Moisture and oxygen molecules migrate into the fluid through all of the rubber components of the braking system Modern brake hoses are much better than they used to be, but they are still semi permiable to oxygen and moisture.
Not changing the fluid leads to a reduction in the boiling point of the fluid, which can lead to brake fade under heavy braking conditions (an emergency stop on a hot day with a fully loaded car for example). Moisture also causes corrosion of the internal parts of the braking system. It also increases the compressibility of the fluid, so the braking effort is less efficiently transfered to the calipers.
It should cost about £60 - £75 to have the brake fluid refreshed at a good indy garage. Hardly expensive every four years really??
Personally, I use DoT5.1 and bleed it thoroughly every 2 years.
PS DO NOT mix DoT5 into a DoT 4 system (it's easily confused with DoT5.1 - always seemed odd to me that they didn't make it DoT4.1).
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