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DOT 3 or DOT4 Brake Fliud

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S

Soup

Jedi Hopeful
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I rebuilt my brakes last season except for the master and servo and used DOT 3 brake fluid. This season my master began leaking. Could this have been caused by the DOT 3 fluid? A freind of mine just told me that he thought the DOT 3 fluids are not compatiable with the rubber components in the british cars. Now that I have just finished rebuilding the master cylinder and the easy to get at parts of the servo unit should I be using DOT 4 rated brake fluid??
 
DOT 3 and DOT 4 are totally compatable and they can be interchanged or even mixed with each other. I prefer not to mix them though.
DOT 5 is silicon based, and will rot anything rubber that has been used with 3 or 4, so that is a definite "nah nah"!
DOT 2 was designed for drum brakes only and is almost obsolete.
The basic differece is the boiling point, I think 3 is better than 4, that's why a lot of racers use it. The problem with 3 is that it obsorbs moisture quickly and moisture in brake fluid is again a big "nah nah". Racers don't care because they change their fluid a lot.
Castrol LMA would be a good choice an I think comes in both 3 and 4, (the LMA stands for Low Moisture Activity or Action or something like that). Either way, you should have no problems with 3 or 4.
 
So it doesn't sound like the DOT 3 is an issue as far the rubber components are concerned but I would still prefer to bleed the system and change it out with the DOT 4. I'll try and find the Castrol LMA in the 4.
 
DOT 4 has a higher boiling point then DOT 3. Castrol LMA is basically a DOT 4 brake fluid. At least according to the info at this site.
 
Hi There Soup;

Just passing thru and thought I`d add my $.02:

According to My Brit Guru Mech; Nothing except Dot 4 min should be used! Should You go Dot 5; Drain Everything to the Bone & Use it "Forever"! Whatever You do; Don`t Mix any of them!!!

Regards, Russ
 
svtmikey said:
DOT 5 is silicon based, and will rot anything rubber that has been used with 3 or 4, so that is a definite "nah nah"!
Click to expand...
Hogwash ! I've had DOT 5 in my TR3A for almost 20 years, and it works great !
svtmikey said:
Either way, you should have no problems with 3 or 4.
Click to expand...
Apparently no longer true (with all new seals), but at one time, "American" brake fluid, either DOT 3 or DOT 4, was not compatible with British brake seals. You had to use 'British' (Castrol or Lockheed) brake fluid.
 
Dot 5 is the only fluid to use. I also have dot 3 in my TR3 since 1982 and no problems. My 250 for 2 years now. Manual and my local Triumph dealers have told me nothing but dot 5. Not only will it not deteriorate natural rubber seals but it will not absorb moisture and have to be change every few years. Not only will you have to rebuild the master cylinder but also the brake calipers and cylinders.
 
I agree, the best most likely is DOT 5, BUT not if you have previously used DOT 3 or 4, then you will run into problems. If everything is new, that is through out the complete braking system, should will be ok.
As far as Dot 4 having a higher boiling point than dot 3. That is not true. However, as I said Dot 3 abosrbs water at a much higher rate than DOT 4. This absorbed water does in fact reduce the boiling point signifacantly and drastically reduces it's performance to lower than DOT4. As I said this is not a problem if you change your brake fluid very often as in the case of racing. BUT in the case of us mere mortals who don't completely change the fluid every 2nd or third race (truth is I don't race at all), DOT 3 is not suitable. I personally have been known not to change my brake fluid for (I hate to admit it) years, DOT 3 is definitley not the choice for me. I use DOT 4 in my GT6, as well as in my Contour SVT, and have always been very happy with it. Let's face it, how many of us actually drive hard enough to boil brake fluid anyway.
 
Point Roberts, WA
Did you get to VanDusen last weekend?
 
I don't wish to dive into the DOT-5 vs. the others debate but have a general comment on the use of DOT-4 vs. DOT-3 in old LBCs.

Yes, there was a time when you had to be VERY careful about which fluid you used. The fluid compositions didn't change, the brake rubber did. Sometime in the mid- to late-1970s LBCs started using rubber that was compatible with both DOT-3 and DOT-4. Prior to that you could ONLY use DOT-4, specifically recommended was Castrol LMA. IF your car still has ANY of its original rubber (hoses, seals, proportioning valves, etc. ) you should stick with DOT-4. IF the car has had all the seals replaced in the last 15 to 20 years... you're probably OK with DOT-3. HOWEVER, I would NEVER mix DOT-3 and DOT-4. The systems weren't formulated to be mixed together. Flush the system so you've only got one type of fluid in there.

DOT-5 is a totally different animal with its own set of problems/traits.
 
Maybe this depends on whether you have dinner or supper each evening... or whether you like butter or sour cream on a baked potato... or if you're boxers or briefs.

FWIW -- I'm: dinner, sour cream, briefs, DOT5.
 
I think its just coincidence, seals fail.

maybe when you bled the brakes, the extra pedal action from bleeding churned up grime and allowed dirt to get between the seals?
 
I enjoy the aspect of this forum, where people share knowledge they have garnered.

There is one item I would like to point out that is perpetuating a falsehood. Most of the sights still refer to non silicone fluids as hydroscopic.

Incorrect. As I learned from a community college physical science professor.

It is hyGroscopic. G, not D. completely different meaning just by changing one letter.

HyGroscopic means it absorbs moisture, almost like a sponge. And when petroleum based fluids absorb moisture they deteriorate.

HyDroscopic means it is compatible with or made with Water.

My 2 cents worth.
 
dklawson said:
Yes, there was a time when you had to be VERY careful about which fluid you used. The fluid compositions didn't change, the brake rubber did.
Click to expand...

I have a not so humble opinion about this.

Everyone seems to forget the amount of NOS parts in the form of rebuild kits still floating around. And don't forget those aftermarket pacific rim parts. They pop up every few months and people snap them up on Ebay, or on sale at their favorite supplier.

It is highly likely some of these kits contain seals that are still natural rubber, or contain natural rubber in the seals, or just one seal in the kit contains natural rubber. Also if the kits are made in South Asia or Indoneisia, you have no idea what the seals are really made of.

US ester formulations of DOT 3 swells and softens natural rubber seals and causes them to fail rapidly. That is probably why your MC started leaking, or, you have a pit in the bore of the MC at the low spot where the seal rests in the bore which is also typical.

So in your LBC, unless you are 1000% sure all of the rubber seals in your brake and clutch system are new synthetic rubbers containing NO NATURAL RUBBER, then you should use ONLY Castrol Crimson, Sterling or LMA fluids, Dot 5, or a brake fluid that specifically states it is safe for natural rubber seals.

Can you flush all the contaminated water droplets out of your system so they will not sit against the seal and cause a corrosion pit? No. Why? Because that water droplet is in the low spot in the bottom of the master or slave cylinder and the exit and bleed hole is in the side or the top of the cylinder. Water sinks in petroleum products.

Disassembly or bench bleeding are the only ways to get the crud and water out.
 
I have been using dot 5 silicone for 20 years in my TR3 after installing all new or rebuilt brake parts. I switched my TR8 to dot 5 when I bought it six years ago after completely flushing the system with denatured alcohol. I try to remember to bleed through new dot 5 every three or four years. No problems out of the ordinary (I had to change the TR3's hyrdraulic brake light switch to the mechanical TR4).
 
Funny you mention the hydraulic brake light switch. That also happened on my Mini after switching to DOT-5. I found this happened to quite a number of people who made this change. Like you, I installed a mechanical switch on the pedal and I don't regret the change.
 
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