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Okay, I've heard varied opinions on D.O.T.3-4 vs. D.O.T.5 brake fluid used on a '55-'56 with Torsion Leveling. Does it ruin the brake light switch or what? I've always had good luck and a hard pedal with 5. Opinions are all over the place on this. Anyone have the practical skinny on it?

IMO, there is no proof one way or the other. Just anecdotal evidence that those who change the fluid and use 5 seem to report having a brake switch failure not that long after the change. Whether the failure is due to the fluid or the switch was on the way out before the fluid change and would have gone anyway is unknown.

The silicone is slightly thinner fluid so conceivably it could seep into the inner workings or there may be an incompatible substance that is damaged by the silicone. I don't know if anyone has ever taken a failed switch apart to see if they can spot what happened.

I don't that it's as much opinion that varies than it is actual experiences. Switch failures have been reported with both types of fluid, but of course that's no surprise - switches fail eventually even if fluid compatibility isn't at question. But the preponderance of reports seem to focus as DOT 5 and being cited more frequently with switches that failure before what people assume should be an acceptable lifespan. Sounds like you're looking for some fundamental scientific assessment - none that I'm aware of.

Why did you move to DOT5 in the first place? I am not aware of any OEM brake supplier for cars or motorcycles that specs DOT 5 and for good reasons.

DOT5 was developed by the military as a fluid for vehicles that are seldom used and stored most of the time. It has a somewhat higher boiling point and is less aggressive to paint. However, DOT5 will expand in high heat and at altitude causing the brakes to lock up. You would have to bleed the brakes in order to release.

There is some degree of air and moisture in ANY brake system. The problem with DOT5 is, the air and moisture remains separate from the fluid and it causes the brakes to exhibit a "spongy" feel.

BTW, are you certain that your entire brake system has components that are compatible with DOT 5? You do not just replace DOT3/4 with DOT5. You either have to have a new, clean system or thoroughly flush your system of any remnants of DOT3/4. The fluids will react when mixed and coagulate.

Therefore, you will not find DOT5 in my garage.

I know it's either or and they're not compatable. I've used 5 on bikes with great results.

DOT 5 (silicone) used in my 56 Executive since 1997 and 30K miles that i know of. The previous owner had converted to DOT 5. I just continued on with it. Swapped out the BTV for a modern unit about 2003.

I've had NO problems whatsoever EXCEPT for the brake lite switch.
Whehter that is due to silicone or not i don't know. A minor inconvenience even if it is due to DOT 5.

When i eleminated the BTV i mounted a MECHANICAL sw. to the brake pedal.

NOTE THE FOLLOWING QUOTE from above post:

" DOT5 was developed by the military as a fluid
for vehicles that are seldom used and stored
most of the time."

The case is made for DOT 5. Verdict: NOT guilty.

There are a lot of assertions going around about DOT 5. I have some first hand experience.

a) Does Dot 5 react with Dot 3 and 4 with damaging results?

Results: When I was in Africa they were always dumping DOT 3 into military vehicles with DOT 5 because they didn't know any better. We never had to do a single repair on a vehicle because of that.

b) Does Dot 5 turn Dot 3 into sludge?

Results: I took a clear plastic bag with DOT 3 and DOT 5 and hung it on the wall for a couple of years. When I was asked about the issue I showed them the demonstration. The DOT 5 is inert and simply sat there separated from the DOT 3.

c) Does DOT 5 cause hydraulic brake switch failure?

Results: I have had such failures with both DOT 5 and modern DOT 3. However, I have a vial of 55 year old DOT 3 and it is not the same as modern DOT 3. The old brake fluid was a Castor oil base while the new stuff is glycol-ether. To my recollection those switches didn't give the frequency of problems in the old days as today. In fact, I don't ever remember having to replace one.

Hope this helps.

I converted to Dot5 when I was redoing the 400. The brake light switch went out and I put a new one in. It went bad also. I cut it open and saw the fluid had gotten beyond the rubber seal and the contacts were burnt so bad they no longer made contact and the switch just could not work. I put in a GM dual mechanical switch. It shuts off the TL when it activates the brake lights.
Problem solved for around $10.,00.

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RogerDetroit, I NEVER said I had moved to D.O.T. 5. I merely asked for information. Fine if you don't care for it, other folks don't seem to feel so strongly though.

My only real concern with DOT 5 is that some folks make the assumption that once they change over it's good forever. Moisture enters the brake system by the same mechanisim (via humidity-laden air drawn in thru the vent each the brakes are applied) as with any other brake fluid. Since the moisture isn't absorbed and dispersed thru the fluid as it is with conventional fluids, it tends to collect in the lowest point in the system where it can do damage. So regardless of which brake fluid type you use, changing it every few (3-6?) years is very good preventative maintenance.