Systems designed for a particular type of fluid ( especially prior to the wide distribution and use of DIT 4 fluids ) should continue to be filled with that fluid
for example in a car that was delivered with DOT 3 fluid , the internal components of the system ( seals , brake hoses, and fitting for example ) were specifically designed and tested for compatibility with the chemical composition of DOT 3 fluid
Because DOT 4 fluid typically contains a different chemical composition, compatibility of system components may be an issue
-- extract from stopcheck--high performance brake systems--
another site that has an explanation of the DOT ratings for brake fluids is
www.tccoa.com>brake fluid
Basically DOT 4 will replace DOT 3
it is more expensive and good practice will require the complete flushing of the system
THE ONLY BRAKE FLUID THAT CANNOT BE USED IS DOT 5 AS IT IS NOT COMPATIBLE WITH THE RUBBERS AND SYSTEM
I suggest that you read the tccoa site as it has a good explanation and offers a clear understanding of the DOT system
Lastly ,if you still have doubts the shop you should be talking to is an accredited brake specialist shop , not your local mechanic that has a limited knowledge of and relies on the sales pitch of the fluid manufacturer
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