I’m adding to this older post rather than continue the discussion regarding Dot 5 fluids that was started in the wheel cylinder installation post.

Doing a search on this site using Dot 5 or Silicone brake fluid will also bring up other discussions on the subject.  From experience I can only warn you to completely clean a brake system thoroughly if switching from one type of brake fluid to another.  For myself, converting to Dot 5 fluid was analagous to switching from points to breakerless ignitions.  Started using these fluids in the early Eighties and have used them on every complete brake rebuild since.  But I do tag each master cylinder lid on each car that has it with a metal label specifying that the system is equipped with Dot 5 fluid.

The US Military uses Dot 5 due to its non-hygroscopic nature.  And as has already been mentioned, it is the factory supplied brake fluid in the Harley Davidson motorcycles.  Scores of Corvettes out there with Dot 5 fluid and I’m aware of at least one dealership that has a program to retrofit those not with Dot 5 with it.  It’s a given that there are many warnings out there not to use Dot 5 in cars that were originally equipped with Dot 3 or 4 fluids simply due to the incompatibility issues between the glycol based and silicone fluids.  Dot 5 is not the cureall for all braking woes but for myself the positives outweigh the negatives and subsequently makes it the brake fluid of choice for myself.

For the vintage car crowd, Dot 5 brake fluid does greatly minimize the degradation that occurs as a result of moisture hydration that takes place when using Dot 3 & 4 fluids.  And the Dot 5 fluids are much more capable of longer intervals between brake fluid changes.  Water pooling with Dot 5 fluid is not an issue if due diligence is taken in not washing down the engine compartment and flooding the master cylinder with water.  Air entrapment is another issue though as pumping the brakes repeatedly can pull air into the fluid and make for a spongy pedal.  I have good solid pedals on all my vehicles but also make it a point to use good brake lines throughout.  Older brake lines will give that spongy feel regardless of the fluid type being used.

A new generation of Dot 5 (5.1) fluids are now on the market which are glycol based rather than the silicone based fluids that have been around for awhile.  The glycol based formula is designed to have less of an air entrapment within it thus making it more suitable for proportioning valve equipped systems.  I haven’t had an opportunity to test this newest generation of brake fluid but with a new (old) car going together, it’s an opportunity I’ll consider.  I haven’t had any issues with the original silicone based Dot 5 fluid so I have to question if I want to add another brake fluid type into the fray.

Not trying to sell anyone on switching to Dot 5 fluids, just letting you know that I use it extensively without any problems.  Feel free to add comments.

Lorena, Texas (South of Waco)

My Harley came with DOT-5, but NONE of my factory Ford cars used it.  In fact, none of the OEM's fill with DOT-5.  The reason being, nothing works better than DOT-3 for an occasional or daily driver.

We can talk about seal-type incompatibility, spongy pedal, water, or air issues if you like.  Brakes are THE MOST IMPORTANT SAFETY ISSUE in every vehicle, and VERY susceptible to law suits.  If DOT-anything was better than DOT-3, believe me, the big three would have it in all their cars.  The whole system only holds ~a cup!  The cost difference isn't very much to an OEM (compared to one negligence judgment in court).

Be careful filling your DOT-3 because it does eat paint.  It easily washes off with water.  If you decide on using DOT-5.1, remember it is still glycol based so have wash-water available.

Consider the mechanics involved with every cylinder application where air is on one side of the piston, and a liquid on the other; master cylinders and wheel cylinders.  DOT-3 is 600 times more dense than air.  That's why it seals so well.  When a seal moves over a bore, the liquid fills all those tiny tool marks and helps lubricate the action.  When retracted, the liquid in those tiny score marks picks up water in the air.  The next time the seal slides over, new fluid mixes with the saturated fluid and it disburses with all the fluid.  Over years of use, the system becomes saturated with water.  There is no such thing as a "perfect" seal and normal brake system pressures operate between 600-2,000 psi.  Serious pressure!

One way to reduce this problem is by limiting piston motion.  Disk brakes don't retract their stroke, which greatly reduces fluid saturation.  Drum brakes can be fitted with "Residual Valves" which always maintains ten psi line pressure.  That stops shoes from retracting all the way, and it keeps the brake pedal (M/C) at the top of its stroke, further reducing saturation.

Dave Dare

Ford 292 Y-Block major overhaul by simplyconnected