Brakes: DOT 5 (Silicone) Brake Fluid

[unclemiltie]

I went to take the 64 Convert for a drive yesterday and found I did not have any rear brakes. I got under it to find the leak and could not see anything. I had my wife press on the brake pedal and saw fluid running out of the brake light switch. I had one other brake light switch failure in a car with silicone where the switch just stopped working.

I use silicone in 7 Studebakers, some of which I seldom drive. I have been using it for around 25 years now. 2 switch failures does not seem bad at all as the switches are easy to change and not that expensive. This one was on the bottom of the master cylinder under the hood, cannot get much easier.

I have also noticed that if a car sits for a year or so, sometimes a wheel cylinder will have a piston stick in it and the brakes will grab as a result. The first time this happened, I tore it apart and found nothing other than dryness. I now just carefully drive the car and frequently press the brakes to get the piston lubed. It will normally lube in a block or so.

Other than that, I have not seen any other issues with silicone.

All this said, I am very happy with silicone. I still remember the days of having to rebuild the wheel cylinders on seldom used cars with DOT 3 every several years and that was a pain.

An occasional switch or dry piston is minor compared to rebuilding a brake system.

I know that are switches out there which will are silicone tolerant, but since I have a supply of the standard switches, I will stay with them for now (CASO comes out!)

And I do not own stock in a silicone manufacturer!

[53commander]

Has anybody used 5.1 yet?

[52 Ragtop]

I think it's been mentioned here before, but a Harley Davidson brake light switch is "supposed" to be OK with silicone fluid.

Jim

[JoeHall]

I think it's been mentioned here before, but a Harley Davidson brake light switch is "supposed" to be OK with silicone fluid. Jim

I wish that were true Jim. I have had a Harley since 2004 that has always had DOT 5, and the rear brake lite switch needs replaced once or twice a year. I have tried "authorized Harley Davidson" switches, and el cheapos, both with same results. Had same experience with a 56J I once had that the PO had converted to DOT 5. The Harley rear switch is same as Stude, and is even inter-changable.

[SN-60]

Someone needs to invent a 'DOT 6' Brake Fluid........a brake fluid with all the nice advantages of 'DOT 5', without the not so nice negatives.

[Rerun]

I have been a fan of DOT 5 for a long time. Part of the changeover is to simply eliminate the pressure switch and install a mechanical stoplight switch on the brake pedal. On Studebakers with swing pedals that is fairly simple. Put pigtails with bullet end connectors on the switch. Make them long enough to go through the firewall and plug in to the connectors that originally went on the pressure switch. No messing with the original harness.

In addition to eliminating switch failure, the mechanical switch is faster acting than a pressure switch. Than could potentially prevent someone from parking their vehicle in your trunk.

[64V-K7]

The trick to extending the longevity of a standard brake light switch using silicone fluid is to fill the switch with DOT3/4 before installing it.

[Dwain G.]

I wonder by what logic DOT 5.1 is NOT a silicone fluid. Seems to me using the number 5.1 implies that it is an improved version of 5.0. Instead it is a synthetic and improved version of DOT 4.

[Son O Lark]

The last switch I bought at NAPA seemed to be newly manufactured. I switched back to DOT 3 at the time because of too many small leaks and switch failures.

[studebaker-R2-4-me]

I would be careful using a silicon based brake fluid in any Hydrovac equipped car. If the membrane fails in the hydrovac brake fluid will be sucked into the intake manifold and you will be burning silicon brake fluid through two or more cylinders. Probably not a good idea.

Personally I am removing my rear differential, hydrovac's, and master cylinders, enough to pump new brake fluid through my system so water retention in the fluid is not an issue anyway.

Allen

[prez55]

If silicone is so good why do no major car companies not use it.

[sweetolbob]

If silicone is so good why do no major car companies not use it.

DOT 5 costs more, DOT 3/4 has a history of successful use and there would be no reason for dealers to try to convince you needed to change it if it was DOT 5.

Why would they change??

Bob

[JoeHall]

The trick to extending the longevity of a standard brake light switch using silicone fluid is to fill the switch with DOT3/4 before installing it.

I understand under NO circumstances should DOT 5 and 3/4 ever be mixed. Matter of fact, getting all the old type out is a major concern in changing over, whether going to or from DOT 5..

[Gunslinger]

Most, if not all performance car manufacturers specify NOT to use DOT 5 in the cars they build. It's very difficult to completely purge the air from a system filled with DOT 5 and the extreme handling that performance cars can be subjected cause the fluid to aerate. That can create air bubbles large enough to act as a clog in the brake lines or at minimum give a spongy pedal feel. In a normal street driven car this may not be an issue but spirited handling has enough potential for problems that many car makers specify DOT 3 or 4 only.