VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc brake fluid

Was raeding the post of snowball41 in the 37-41 forum dealing with the brake system. I have replaced the DOT3 with DOT5(silcone) in my 41. I would assume that the use of DOT 5 has been discused in this forum, wonder if I might get a quick update.

Quick update - don't use it.

I have had Dot 5 in two of my cars for over 25 years with no problems. I do bleed and refill the system every 10 to 15 years if at all.

Stick with DOT 3...actually you should change DOT 5 more often...wait I don't change it anyway...my bad

I actually change it out about every two years. Then I only have 1 car, am retired and need to stay in the garage as much as possible.

How is your wife these days ?

About the same - which isn't that great.

Never thought of using DOT 5 in the 41 but have used it in my 78 Vette I race for years and it works great.

Trying to think of something good to say about DOT 5 . As far as some significant advantage over DOT 3 . But ah , I got nothing .

While doing the mechanical restoration on my 40 1/2 ton pickup, I sleeved the wheel cylinders with stainless steel and installed all new stainless steel hydaulic lines. Since everything (except the master cylinder) was new, I thought I'd try DOT 5 fluid, as it does not absorb moisture. I filled and bled the system and found that three of my four wheel cylinders were leaking fluid. I re-polished their bores to a chrome-like finish, and I still had some leakage at one cylinder. I re-sleeved it a second time and still had leakage, so I flushed out all the DOT 5, rinsed with DOT 3, then filled the system with new DOT 3. Now, after two years and around 5000 miles, I've had no leakage or any other issues. I'm not sure what the problem was with the DOT 5--perhaps you need to use a particular type of rubber cup in the master and wheel cylinders with DOT 5. Anyway, the takeaway for me was that I'll leave the DOT 5 for new cars and stay with DOT 3 in the vintage cars.

Mark

Dot 5 is compatible with the natural rubber cups. Compatibility, including swell are a requirement for any brake fluid. I worked in the group that did most of the initial testing when Silicone (Dot 5) brake fluids were developed by Dow Corning. It had to pass all the standard tests for brake fluids before it could be sold. The company I worked for was the largest (think still is) brake fluid supplier in the world.

While Dot 5 fluids have some advantages over Dot 3 and Dot 4 fluids, they also have some disadvantages. Tendency to leak and deterioration of some brake light switches are the two most often seen. Another is even though they don't attract water like the glycol ether based fluids, that does not mean that water can not enter the brake system. When it does the water droplets can cause problems because it was not absorbed into the fluid. Frozen ice balls blocking brake system passages was a real problem in colder climates.