Hi guys, Please give me some insight on what brake fluid you think would be best for my new resto. It is a '72 442 350 4-speed with power disc. Standard or Silicone? I want to drive this car somewhat, but certainly not every day. Thanks in advance, Dave.

 

Dave, either will work and it comes down to a preference. There are some advantages to silicone (DOT5) and I do use this in the cars I have restored given the little use they get. Silicone will not damage paint, it will not absorb moisture and it has a higher boiling point. I would think that the first two points are what is important. DOT3 absorbs water and can cause corosion in the brake system especially on cars that sit for extended periods of time. If you do some regular maintenance and flush the brake fluid from time to time, then this problem is not an issue and DOT3 should work just fine. Just keep it away from the painted areas.

Keep in mind that regular fluid (DOT3) and DOT5 are not compatible and should never be mixed. If your existing system has DOT3 and you don't plan to rebuild the system, then you will need to stay with the DOT3. If you rebuild the entire system and replace all of the hoses, calipers, master cylinder and wheel cylinders and either completely flush the lines and distribution block or replace them, then you could go with DOT5. Be careful with new master cylinders as they can have a little standard brake fluid in them from assembly. You'll need to clean that out if you want to use DOT5.

DOT5 is also a little thicker and I have noticed that the master cylinder really needs to be bench bled to get the air out. Once it is bled along with everything esle, it works fine. If you were not aware, DOT5 is also considerably more expensive than standard fluid so give that consideration too. I recenly paid $29/qt at a local NAPA. I know this store isn't the cheapest around but it's the most convenient for me so I am willing to pay a little more.

Where in MD are you located?

Brian

 

That's a question that should generate a vigorous "discussion."

Silicone brake fluid won't rot your brake components, but it has a distinctly different feel than the regular kind.

Silicone feels just a bit "spongy" due to the fact that it absorbs tiny amounts of air, rather than water. Some people say they can't feel it, or don't mind it, and if you've got a car that is hardly ever driven, it's probably the best, because you can pretty much ignore it.

I don't like it myself, and would recommend the regular type for any car you actually drive, and suggest that you change out the brake fluid every year or two, to remove the old fluid that has absorbed water.

- Eric

 

I use a DOT4 like GS610 and prefer a fluid that won't let absorbed water vapor puddle inside lines and cylinders and cause havoc. Silicone fluid also leaks through fittings easily and it can be a real hassle to seal the system.

 

DOT3 absorbs water by design and it is meant to reduce corrosion in the brake system by pulling moisture away from the systems components – but you need to maintenance it like so many other things to keep it in tip top shape. If you don't its ability to take up moisture becomes saturated and there lies the problem.

I use the dot3 and never have any issue. It was good back in the day and still is now.

It is your preference but I hope this helps clear things up.

 

Here is what happens when you mix Dot3 and Dot5, Sludge.
A few weeks ago a person accidentally added approximately 4 ounces of Dot5 to my Dot3 1971 Cutlass (into the front reservoir). The rear reservoir got a much smaller amount of less than 1 ounce.


In the attached picture you may be able to see part of the difference. I took a paper towel and dipped it into each reservoir, down to the bottom. The one on the left is has the 4 ounces of Dot5. The right is the reservoir with less than 1 ounce of Dot5 mix. What you can't see from the picture is the thickness of the sludge. The bottom quarter inch of the reservoir was all black sludge. I used a flashlight and looked down into the two bowls. Front looks scummy and muddy, the Rear you could actual see in half inch or so.


My brakes lasted about 30 days from Dot5 until the peddle hit the floor. the local brake chain is afraid to bleed the brakes because the nuts have not been moved in 40 years. Looks likes its going to be a long weekend.

 

Dot3 and Dot5 sludge.

 

I have heard cars with Dot 5 can have issues if they ever use their cars in higher elevations. The small air bubbles previously mentioned become larger because of lower atmospheric pressure at higher altitudes. I have driven my old cars in the mountains of North Carolina. Since I live in South Florida at near sea level, that pressure change can be significant. I don't want posible brake failure or reduced brake efficiency while coming down a mountain road.

I have never had an issue using Dot 3, but agree, it needs to be flushed every few years. That is no different than coolant or regular oil changes.

 

I use DOT4.

DOT5 I use in my Hayabusa because it was factory.

 

Both have their good and bad points, just make sure that you don't mix them and make sure that you settle any component compatibility issues. I have heard that silicone fluid can swell the rubber in a dot 3 system. I avoid the issue and just use dot 3. period.

 

And, lest anyone get the wrong idea, remember that DOT 3 and DOT 4 are basically the same thing, with slightly different heat ranges.
That means that DOT 4 will go an extra 50° or so before it boils, but DOT 3 will stay thin down to something like 50° below zero.

Most folks who drive antique cars will lose nothing by using DOT 4, but since they don't heat up their brakes to hundreds of degrees, they won't gain anything either.

I use whatever's cheaper.

- Eric