Packard Forum Index » V-8 (1955-56)

ok. With silicone the moisture will collect at the lowest points of the brake system. i.e. the wheel cylinders.

So then with silicone the wheel cylinders take the compromise from moisture. Bleed wheel cylinders. OTHERWISE, the moisture is suspended thru out the entire brake sytem thus cauing corrosion thru out the entire brake system. NOT just the wheel cylinders.

Let me put it another way, i'd rather corrosion occur in one part of the system than to occur thru out the entire system. No???

There is another point:
I don't claim to be any kind of a chemical eng'eer. But it is my understanding that DOT 3 ATTRACTS (or tends to wick) mositure while DOT 5 has no such moisture attraction.

I do claim to be some kind of chemical eng'eer (opinions differ on what kind), so I'll give it a try

DOT3 - ether based. boiling spec 401 F/ 284 F dry/wet

DOT4 - ether based. boiling spec 446/311

DOT5 - silicone based. boiling spec 500/356

DOT 5.1 - ether based. boiling spec 500/356

So what?
DOT 3,4, and 5.1 are compatible with each other.
DOT5 is not compatible with the others. you know that.

higher boiling point matters if you are braking hard a lot.
the boiling point goes down as the moisture is absorbed as shown with the specs. probably not an issue with most of our Packards.

DOT 3,4,5.1 absorb water up to a point - this is not a bad thing in the short term, it prevents free water. They all damage paint. Longer term it means they need to be flushed.

DOT 5 absorbs very little water. It does absorb air more easily and clears air bubbles less readily. Thus the tendency to more compressibility and softer pedal.

Free water can quickly cause local corrosion

Agreed, flushing the system and putting in fresh fluid is a smart move every 3-4 years. Nothing is truly maintenance free!

I believe DOT 5 is standard equipment on Harley Davidson.

all I can say is that I used silicone is my 56 400 sat for 10 plus years, and no water, no gunk,,,,,clean as could be..when I took it off car,

Let me put it this way:

I would not convert an already existing IN GOOD CONDITION DOT 3 brake system to Dot 5. I would continue on with the DOT 3 evem at a future rebuild.

However, i would not convert an alredy existing DOT 5 system back to DOT 3 unless some conspicuous and irrefutable damage hadbeen done bythe DOT 5.

To date, there is no solid evidence to discourage the use of DOT 5.

I will say this about the hydraulic brake switch problem.

The ones I had problems with were replacements. I scrounged up an old one once (35 120) and it still worked fine. But as I recall, it too became intermittent, thus my suspicions about modern DOT 3.

The system is not a good one because the switch is load bearing. I think the best way of using an existing switch is to use a 6 volt relay. That will reduce arcing damage inside the switch and convert it to a sensor.

I don't think flushing a DOT 5 system is necessary. The owner's manual for my modern car says nothing about flushing the brakes either. There are thousands of Toyota Corollas running around with 300,000 miles on them with the original fluid. The bigger problem is rust on the surfaces that are exposed to air because the shoes aren't worn yet. When DOT 5 is used a corrosion retarding film is created when the cylinders retract. With DOT 3 a rust zone is created.

Another aspect of this issue is that, if water intrusion was such a big deal, modern master cylinders would not be vented they would have an expansion chamber. You would see reservoir caps with a spring loaded diaphragm to handle compensation and replenishment. There's a great aftermarket gimmick.

So I would prefer DOT 5 on drum brakes hands down. Especially because during assembly the exposed surfaces are coated with a corrosion retarding film. DOT 3 creates a rust promoting film.

Some suppliers I used said they didn't want DOT 5 used with their parts. Okay. And some of those parts failed anyway so they were just playing the blame game.

Tim, not sure if you missed this or not, but the relationship between hydraulic brake light switch failure and DOT 5 is almost exclusively associated with the unique switch used on the 55/56 cars with TL - preponderance of evidence on this (only) switch and DOT 5 is extensive. In my experience there is no supportable finger-pointing with regard to other brake light switches failing prematurely with DOT 5.

As I stated, I have had suppliers state that DOT 5 was not to be used with their parts. Okay. So I used DOT 3 and the parts failed anyway.