View Full Version : TR6 Dot3 or 4 on a 75 TR6

09-23-2008, 04:48 PM
Hi all,
I'm about to order rebuild kits for my clutch slave and master for my 75 TR6. I have already replaced my brake master from Apple hydraulics last year. While wondering why my clutch master wasn't pumping down to the slave, I found it leaking under my dash at the pedal. That's when I noticed that the old brake master was leaking through to the brake pedal before I replaced it.
Now I'm assuming the booster has some in it too. At time i do lose brake pedal. Could this be related to the booster? The note that came with the brake master stated only to use dot3 or 4 but not dot5. I believe I used dot3 from Napa. I'm really confused on this 3 or 4 thing. With the new kits which can I use. I know I probably should be use the Castro LMA but can not find it here in town. Will 3 hurt the new rubber? Also....is there really that much of a difference in the kits from the 4 parts suppliers? Please set me straight.. thanks

09-23-2008, 05:26 PM
Try Dot 5

09-23-2008, 06:10 PM
Floyd, this subject never fails to stir up contraversy. But if you want DOT 4, NAPA sells it. "NAPA Premium Brake Fluid" "exceeds DOT3 and DOT4, LMA (Low Moisture Absorption)". Part #40-012.

09-23-2008, 06:44 PM
Aloha Floyd,

Here is run down on US Dept of Transportation brake fluid types and standards. The choice of which type to use is up to you. I personally use DOT 5.

These type fluids are hygroscopic meaning they have an ability to mix with water and still perform adequately. However, water will drastically reduce the boiling point of fluid. In a passenger car this is not an issue. In a race car it is a major issue because as the boiling point decreases the performance ability of the fluid also decreases.

Poly glycol type fluids are 2 times less compressible than silicone type fluids, even when heated. Less compressibility of brake fluid will increase pedal feel. Changing fluid on a regular basis will greatly increase the performance of the brake system.

FLUID SPECIFICATIONS All brake fluids must meet federal standard #116. Under this standard is three Department of Transportation (DOT) minimal specifications for brake fluid. They are DOT 3, DOT 4, and DOT 5.1 (for fluids based with Polyalkylene Glycol Ether) and DOT 5 (for Silicone based fluids).

MINIMAL boiling points for these specifications are as follows:

Dry Boiling Point Wet Boiling Point
DOT 3 401F 284 F
DOT 4 446 F 311 F
DOT 5 500 F 356 F
DOT 5.1 518 F 375 F

Silicone-based fluids are regarded as DOT 5 fluids. They are highly compressible and can give the driver a feeling of a spongy pedal. The higher the brake system temperature the more the compressibility of the fluid and this increases the feeling of a spongy pedal.

Silicone based fluids will not damage painted surfaces they are also somewhat common in show cars.

Silicone based fluids are non-hygroscopic meaning that they will not absorb or mix with water. When water is present in the brake system it will create a water/fluid/water/fluid situation. Because water boils at approximately 212 F, the ability of the brake system to operate correctly decreases, and the steam created from boiling water adds air to the system. It is important to remember that water may be present in any brake system. Therefore silicone brake fluid lacks the ability to deal with moisture and will dramatically decrease a brake systems performance.

Source, www.afabcorp.com (https://www.afabcorp.com)

09-23-2008, 06:46 PM
Neither - go with silicone, DOT 5.0

09-24-2008, 01:09 AM
In a passenger car this is not an issue. In a race car it is a major issue because as the boiling point decreases the performance ability of the fluid also decreases. I'm sure they mean well, but that's kind of twisted. It's quite possible to boil the fluid even in a passenger car, if it's driven hard or down long grades. And when the fluid boils, you have sudden, total loss of brakes. Obviously this is a condition to avoid !

Glycol fluids actively "suck" moisture out of the air. There is also research indicating that they can absorb not just water, but salt (!!) right through flexible brake lines. The result is quite corrosive, especially to the mixed steel/aluminum components found in Girling drum brakes (TR6 rear brakes).
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]When water is present in the brake system it will create a water/fluid/water/fluid situation. [/QUOTE]Although not strictly true; I'll agree that if you plan on adding water to your brake system, you probably shouldn't use silicone. You probably shouldn't be driving either !

Obviously, I'm a big fan of DOT 5. Been using it in all my cars (and a Winnebago) for many years. While I agree it's not perfect, what I've found is that my rate of hydraulic problems has gone down markedly, ever since I started using DOT 5.

Just one example, I converted a 1980 Chevy to DOT 5 when it had it's first hydraulic problem, at about 80,000 miles. Replaced the leaking caliper, left everything else as it was except for bleeding until I got clear purple at all 4 corners. 170,000 miles later, it still had the original hoses and seals everywhere (except that one caliper), and still stopped just fine.

09-24-2008, 05:15 AM
Hi There Floyd;

Man; If you go back a couple of months, There is a ton of conversation about this same question.

In my opinion; "DOT 5" is the best cause it takes a heck of alot more heat than Dot 3 or 4.

You can (Not Recommended) mix Dot 3&amp;4 but if your going to go the "Best" route, "DOT 5"; Flush entire system &amp; "DON`T" mix any Dot 3 or 4 with the "DOT 5". Start clean from the beginning.

Best Wishes,


09-24-2008, 03:13 PM
Well.....this is just a car for weekend and evening cruises around town. It really does need work before it goes to far out of town. So I'm really just looking to get it functioning for that purpose. The tag I got with my new master brake cylinder ( rebuilt from Apple)did state not to use silicone brake fluid, only dot 3 or 4. I guess my question was....what can I, or can't I, use in my car. I've only had this car for a few years so what is original and what isn't as far rubber parts...i don't know. With these new rebuild kits, what is safe to use with them. Do I need to make phone calls and ask? Thanks again for the help.

09-24-2008, 04:16 PM
If you are going with DOT 3/4; then to be safe I would suggest using Castrol LMA. There have been Girling seals in the past that were not compatible with other brands of brake fluid (and the lack of compatibility was not reflected in the DOT rating).

But most likely it doesn't make any difference. Seems like all seal makers have been using modern synthetic rubber for at least the last 20 years or so; which stands up to all DOT-approved brake fluids.

And if you do go with glycol, I would suggest changing it every 2-3 years (with fluid from a freshly-opened bottle), to try to hold down on how much water is in it. Even if you don't care about boiling point, the water promotes corrosion.

09-24-2008, 04:41 PM
One more question - is all DOT fluid synthetic? Are some synthetic and others not? Marketing fluff or a real consideration?

Andrew Mace
09-24-2008, 06:51 PM
One more question - is all DOT fluid synthetic? Are some synthetic and others not? Marketing fluff or a real consideration? That appears to be the case more and more. I bought a bottle of Castrol GT/LMA a couple months ago, and it's the first time I'd seen the word "Synthetic" prominent on the label. And a few weeks ago, I scanned the brake fluid shelf at a local Auto Zone and saw that same word prominent on almost every brand of brake fluid.

I'm not a chemist, so I've NO idea what "synthetic" means here. :wink:

09-24-2008, 07:07 PM
I'm not a chemist, so I've NO idea what "synthetic" means here. :wink: Basically nothing.
Castrol won a court case a few years back, in effect saying that anything that had been through a modern refinery (hydrocracked) could be called "synthetic".

09-24-2008, 08:11 PM
I too noticed that the Castrol Brake Fluid LMA bottles all now have Synthetic on the front label.
I did not buy it until I found out what was up. This store was the only local one I could find Castrol LMA.
Are the two fluids the same?
This branding issue just showed up in the last 3 months.

09-25-2008, 11:23 AM
Beats me, I never touch the stuff myself.

09-25-2008, 02:29 PM
I have it from a reliable source that the deposits of naturally occurring DOT 3 and DOT 4 brake fluids have been depleted. We now have no choice but to synthesize the stuff. The good news is that the demand for it has decreased due to the increasing preference by many for DOT 5.1 and DOT 5; so we should not expect a dramatic price increase for the synthetic versions.