Will mixing different types of brake fluid ruin my master cylinder?

Dear Car Talk

Dear Car Talk | Aug 01, 1992

Dear Tom and Ray:

At the auto parts store, I've seen brake fluid listed as both DOT 3 and DOT 4. Are these two compatible? I've been told by a friend that these two should not be mixed in the brake master cylinder, or it will turn into a sticky mess. Could mixing these two fluids cause brake failure?

RAY: Not to worry, Hank. You won't cause any problems if you mix these two brake fluids.

TOM: Actually, what we worry about more is people putting stuff other than brake fluid in the brake fluid reservoir. We've actually gone to the trouble of putting labels on our customers' cars that say "Brake fluid only. Do not add motor oil, antifreeze, power steering fluid, transmission fluid, windshield washer fluid, carbonated water, or Fellipo Berio Extra Virgin Olive Oil."

RAY: The two fluids you mentioned, DOT 3 and DOT 4, are the most common grades of brake fluid (by the way DOT stands for Department of Transportation). And the difference between them is just the boiling point. DOT 3 is fine for normal cars and normal driving. DOT 4 has a higher boiling point, and is for vehicles that do a lot of mountain driving, high speed braking, or trailer towing. The higher boiling point prevents the brake fluid from boiling when the brakes are used excessively.

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