Brake Fluids

By Brandon Baldwin

There are two types of brake fluids: glycol based (DOT3 & 4) and silicone based (DOT 5).  Glycol based fluids actually have a slight bit of water content to them, DOT 4 less than DOT 3.  These are hydroscopic too!  This means that they can pull moisture right out the air.  Unfortunately, the more moisture contamination, the lower the boiling point of the brake fluid.  Anyone that has been hard enough on their brakes to boil the brake fluid will attest to the scare of the brake pedal going to the floor!  Hey, any vapor compresses where fluid doesn’t.  Therefore, it’s recommended by many manufacturers to replace your brake fluid every 3 to 5 years.  Have you ever seen brake fluid that is so old it looks like a gel instead of baby oil?  This is far past due for changing.

DOT 5.1 is still glycol based, but has a higher boiling point than the others of 270 degrees.  And these can be mixed.

Then there is DOT 5: NOT to be mixed with DOT anything else!  Although it’s boiling point is 260 degrees, it doesn’t pull water out of the air.  Many antique car owners have been switching to DOT 5 so they don’t need to change it nearly as often (or in some people I’ve talked to, never).  But, if you choose to change over, you must flush ALL other DOT fluid out of the system.  Not only wait to see the purplish fluid come out of your bleeders, but keep going, and go a little more.

If you decide to do this, start bleeding at the brake farthest from the master cylinder and work your way to the closest one last.  Good luck.