Conversion to DOT5

[prb51]

Given the experience of some here I did a conversion to dot5 without cleaning the system first.
My brakes have all been redone with about 10k on them and the masters/slave.
I got a gallon of the purple stuff from a gent on ebay who is selling 1000 gls of military spec dot5...he wants $75 for one but occasionally puts a gallon up no reserve and I got it for $32.
I wanted a good amount as I planned on over bleeding to purge the system which I did (with the wife pumping and doing her cross word puzzle). I suspect I went thru at least a quart and a half.
I used two plastic bottles that I filled the night before and let sit so I could see any bubbles.
I used a small funnel to pour into the resevoir so as to avoid splash/bubbles.
Worked ok.
Clutch fine and engages at the same spot.
Brakes were just a hair soft at first but after a 25 mile ride, bumps, and a few quick stops firmed right up (not unlike my experience with dot4).
No leaks or issues upon return and inspection.
I'll let you know if anything comes up.

 

[MadMarx]

Be sure to change this stuff yearly because it will collect water at places you'll never got an idea about.
This stuff was made for slow vehicles never made for fast sports cars with hot brakes. DOT 4 or 5.1 should be the way to go for sportscars.

Good luck!

Chris

 

[prb51]

chris,
Thanks for the tip. I suspect living in the Arizona desert will help with the moisture issue.

 

[RonMacPherson]

I am going to disagree with Chris on this.

In his climate, it might be a good idea to do a small rebleed every year, but in Arizona, should be no need. Also you may want to do a rebleed in a couple of months as the air bubbles will have "pocketed" by then.

I had Dot5 in the brakes of one of my 6's here in Hawaii for about 6 years and no problem with the brakes(body rusted off0.

 

[prb51]

I just drove again this evening and the brakes seem exactly as they were with dot4. Firm and stopping straight from 70 mph ( I live off a dirt road that t's a 65 mph State rd.)
I wonder about the heat issue.
This is military (came in a US Army 1 gal. can) dot5 made by Corning. All of the US Army vehicles use this stuff.
Huge tank carrying trucks/trailers that carry M1 tanks (70+ ton each load) use dot5 fluid and I suspect their brakes might heat up a tad.
Has dot5 improved and has an old rep for not dealing with heat due to the water issue or what?
Ron, I ran so much fluid thru the system after letting the purple 'breathe' like a fine wine I'd be surprised if there was much/any air in the system. My good and lovely wife got a great one leg work out.

 

[RonMacPherson]

What Chris is alluding to, and what I have found out is the miscibility of the Dot5 is more porous than Dot4(hope I am phrasing it properly).
What happens is when the Dot5 is being installed, being moved quickly without getting a chance to settle, it will form miniscule air bubbles. And they will coagulate when settled and form larger bubbles. I have on several cars installed Dot5, driven for a week, then bled the systems again and pedal is slightly better. But if you take your time with the install and initial bleed, should be no problem...

And the Dot 5 does NOT eat paint!!!

 

[prb51]

Ron,
I took care, after advice from this great body of sage individuals, not to induce air but who knows. If I have to bleed it again who cares, the wife needs her other leg exercised.
The only reason I've done the switch is to save my paint. On the 3 when you have a nice leak it travels down the firewall and the pedal bodies stripping great gobs of paint and everything must be disassembled to clean up (remove masters, pedal box, pedal assemb., firewall cutout etc). I've had to do it twice in 4 years (two sep leaks) but everything is tight now so I wanted to stay ahead of the power curve.
One of my favorite pictures to examine on ebay TR3's is the master cyl area and always, always you see bubbled up paint or rust out from prior leakage.
Not anymore!

 

[TR3driver]

Has dot5 improved and has an old rep for not dealing with heat due to the water issue or what?

Just one of the many myths about why DOT 5 is "terrible", IMO. When Dow Corning sold it for use in general passenger cars, they guaranteed it for 10 years or 100,000 miles of operation.

Here are some interesting "real world" test results, presented at the 1974 SAE Automotive Engineering Congress :

 

[prb51]

The 1 gallon US Army metal can of dot5 I picked up on ebay is made by Dow Corning so I guess they still have the military contract.
Interesting info.
So why don't 'racers' like the stuff...the moisture lack of absorbtion thing?

 

[Don Elliott]

I think racers don't like it because they change it so often and with 10 or 20 minutes between a practice session and their race, they don't have hours or days to let the bubbles rise out of it.

For my 1958 TR3A, I have used silicone brake fluid since 1990, driving city, highway and freeways a little more than 100,000 miles. The braking effect was always like previous fluids and the same as other more convention modern cars. I only once had a problem. That was when I spent a week at an altitude of 10,000 to 12,000 feet during VTR in Colorado in 2001. I think that bubbles emerged from within the fluid with the reduced ambient air pressure at those altitudes. After we bled them, they were fine. I would guess that during the last 18 years, I've changed the silicone fluid about three times. I keep it topped up and that might be equivalent to a full change, as far as fluid volume goes. I never had any issues with silicone fluid related to humidity or water absorption and the lubricating properties have kept all the caliper and brake pistons free and they never rusted or became stuck with rust or crud as they might with other DOT 3 or DOT 4 fluids.

 

[TR3driver]

So why don't 'racers' like the stuff...

One criticism that I've heard is that silicone becomes more compressible near it's temperature limit. One of the SAE papers has a chart showing as much as 3% compression at 1200psi and 500F. There are "racing only" fluids that handle extreme high temperatures better; as long as you change them every season (before they have a chance to absorb moisture).

BTW, a comment about the "minimum wet boiling point" specification : This is not the lowest possible boiling point, but rather the lowest value allowed after a specific amount of water is added. The "real world" test results show that glycol fluids can easily have MORE than the standard amouit of water in actual service, resulting in even lower boiling points than the stated "minimum".

 

[aeronca65t]

Personally, I use DOT-5.1 (glycol) mostly for convience in my race car.
In a pinch, I could add DOT-4 with no problem.
And DOT-5.1 has not boiled off in my car at least.

But I'll be honest.
I have not really studied silicon brake fluid much.

Here is something (shortened, edited version below) I just read regarding silicon brake fluid.

The source is Mike Nixon, the well-known motorcycle tech guru at ~The Motorcycle Project~

<span style="font-style: italic">.......Silicone fluid was able to withstand the most heat of any brake fluid, so it earned a reputation as a racing brake fluid. However, silicone brake fluid has properties very different from glycol fluid, and has its own pros and cons. On the advantage side, silicone fluid will not harm paint or plastic, and does not aggressively attract additional moisture as glycol fluid does. On the disadvantage side however, silicone fluid aerates easily. Harley-Davison, one of the sole current OEM users of silicone fluid, warns buyers to let the fluid sit at least an hour before using it. The trip home in the saddlebag is enough to aerate silicone brake fluid until it looks like a freshly poured soft drink. Silicone fluid is also slightly more compressible than glycol fluid, does not change color to tip the user to its moisture content, and worst of all, neither accepts or disperses moisture, making systems using it more corrosion prone, and requiring more frequent fluid changes. Silicone brake fluid also lacks glycol fluid's naturally occuring lubricity, making it incompatible with the mechanical valving in some antilock braking systems.

The higher boiling point of silicone brake fluid, the reason for its development and at one time its strong suit, is now academic, since D.O.T. 5 glycol (5.1) fluids are now widely available.</span>

He goes on to say that there will continue to be debates on silicon vs. glycol brake fluid, but clearly states his preference to glycol.
I realize that the paint issue is a big deal on restored street cars. To racers like me, it's less of an issue.

 

[TR3driver]

The trip home in the saddlebag is enough to aerate silicone brake fluid until it looks like a freshly poured soft drink.

Hmm, I'll have to loan a can to one of my biking buddies and try that. (I'd do it myself, but gave up riding many years ago.) But after vigorous shaking by hand, or riding hundreds of miles in the trunk of my TR3A, SBF looks like SBF.

And IMO the stuff about corrosion is simply not true. Glycol is far more corrosive than silicone; especially in systems like our with steel pistons riding in aluminum bores.

 

[mallard]

The only problem I had using DOT5 in my TR6 for the last five years was with my clutch slave cylinder. The pedal got soft then the cylinder started to leak. All the rubber seals in the cylinder turned to mush. It may have been a batch of bad rubber. I never had any problems with the brakes.

 

[prb51]

Keith,
How long after your conversion did the slave start to leak/appear soft?

 

[MadMarx]

So let's hear what Castrol has to say about:

++++++++++++++++++++++++++++++++++++
We do not recommend using DOT 5 fluid in any racing applications. DOT 5 fluid is not hygroscopic, so as moisture enters the system, it is not absorbed by the fluid, and results in beads of moisture moving through the brake line, collecting in the calipers . It is not uncommon to have caliper temperatures exceed 200 F, and at 212 F, this collected moisture will boil causing vapor lock and system failure. Additionally, DOT 5 fluid is highly compressible due to aeration and foaming under normal braking conditions, providing a spongy brake feel. DOT 5 fluid is best suited for show car applications where its anti-corrosion and paint friendly characteristics are important.
+++++++++++++++++++++++++++++++++++++

As most of the TR vehicles do has an atmosphere open system - moisture DOES enter the system.

As you mention military vehicles:
They are slow, and does stand around a lot. So for them a DOT 5 would be perfect.

 

[MadMarx]

BTW...I recommend Porsche brake fluid....the best of the best.

 

[prb51]

MM,
An M1 70 ton tank will do over 50 mph. I suspect bringing 70 tons to a quick halt might warm things up.
Anyways got your point.
Does Castrol make a dot5?

 

[mallard]

The rubber componets in the slave cylinder lasted four years. It was as though they were dissolving. If you touched them they would turn your fingers black. And they were all swelled. (not my fingers) I did rebuild the cylinder with todays parts but wonder if this will happen again. The cylinder only had around 2500 miles on it. I was not driving the car much because I just finished a three year body off restoration and was enjoying my brand new car.

 

[prb51]

Keith,
Well, I'll just wait and see. I guess just the quality of the internals. I don't think I'll mind a leak so much now that it's not paint remover.
Thanks.
Anyone else have this kind of issue?