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Use the right brake fluid.

BFS, provided under MIL-PRF-46176B and also known as DOT 5 brake fluid, replaced the old brake fluid. V-V-680, also known as DOT 3 or DOT 4. BFS is more stable at high temperatures and won't absorb water.

So, when it's time to add brake fluid to a master cylinder containing BFS, make sure that what you add is also BFS. A combination of BFS with old fluids will still absorb water, creating corrosion and deposits that cause stopping problems.

Mixing old brake fluid with BFS can also lead to seal leakage, because the seal protective properties of BFS are diluted by the old stuff.

So how do you tell which fluid is which? Go by the color, if you can. BFS is purple or blue in color, though the dye that gives it the color can break down. Then the fluid in the master cylinder becomes brown or amber.

There's no cause for alarm if the color varies--the BFS is still good. But if the color is not purple or blue, you can't tell what kind of fluid is in your brake system.

BFS + Unknown Fluid

Try mixing a few tablespoons of the unknown fluid with a little BFS. If the two mix, the unknown fluid is BFS, too. But if the two fluids separate into layers, the unknown stuff is DOT 3 or 4. Your vehicle needs to have the brake fluid changed.

Unknown Fluid + Water

Put some of the unknown fluid in a jar with a little water and shake it. BFS does not mix with water, and you'll see distinct layers. DOT 3 and 4 fluids, on the other hand, do mix with water and remain mixed. You won't see separate layers.

KEEP THIS INFO ABOUT SILICONE BRAKE FLUID (BFS) IN MIND ...

... ESPECIALLY WHEN THE TEMPERATURE HITS TRIPLE DIGITS.

BFS is purple or blue

Check brake fluid for color

HERE ARE TWO OTHER WAYS TO TELL WHAT KIND OF BRAKE FLUID YOU HAVE ...

Mix? Both BFS No mix? DOT 3 or 4

Mix? DOT 3 or 4 No Mix? BFS

DOT 3 OR 4 FLUIDS SHOULD BE FLUSHED

SEE YOUR TM FOR DETAILS.
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Copyright 2005, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Title Annotation:aircraft maintenance
Publication:PS, the Preventive Maintenance Monthly
Article Type:Brief Article
Geographic Code:1USA
Date:Jul 1, 2005
Words:356
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