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MG MGB Technical - Dot4 or Dot3 Brake fluid

Recently my brake pedal on my 74B got rear low and was almost not braking at all. I tried to buy Dot4 for it but my auto parts store was out for a long time. With the master cylender empty I finally top it off with some dot 3 that I had. Is this going to damage my system? I know dot 4 is called for and that dot 4 is synthetic. I presume dot 3 is petroleum based. Will they be ok together til can bleed the system with dot 4? I only know that #4 has about a 10% higher temperature rating than #3. Any one know anything about this? Bob
Bob Ekstrand

DOT 3 and DOT 4 are both glycol based fluids. Dot 4 is Low Moisture Absorbsion fluid, not synthetic. DOT 3 and DOT 4 fluids are supposed to be compatible. Cheers - Dave
David DuBois

FWIW, I bet the cap on your master cylinder specifies DOT 3 fluid, it does on my 74. I'm not sure DOT 4 fluid existed in 1974.

Clifton Gordon

since MG braking system use naturul rubber Dot 3 eats away at wheel cylinders rubber, master cyclinder rubber that dot 4 must be used. if dot 3 is used you have contaminated the system. dot 4 can be order thru Moss and various auto part stores. don't be surprised when the system fluid turns black from deteriation.

Mike, can you show or link any MGB publication that reads, DOT 4 brake fluid must be used in an MGB or a MGB publication that reads DOT 3 fluid must not be used? I would like to see that information in print. FWIW, I have tried DOT 3 brake fluid in an MGB and nothing happened other than the brakes working normally, fluid didn't turn black.

Clifton Gordon

I kind of thought the natural rubber and brake fluid incompatibility issues were a thing of the past. That is, I thought the natural rubber went out of fasion. I believe I remember seeing strong admonishments in manuals or in boxes of brake parts, and seeing the notes on the cap warning to use only Girling Crimson brake fluid, but that was from my 57 Healey 100-6 and 59 Triumph TR3. Its been a while since Ive seen a can of Girling Crimson.

C R Huff

Listen CR for he speaks the truth RIC

The following is quoted from Federal Standard #116,

"S4. Definitions.
Brake fluid means a liquid designed for use in a motor vehicle hydraulic brake system in which it will contact elastomeric components made of styrene and butadiene rubber (SBR), ethylene and propylene rubber (EPR), polychloroprene (CR) brake hose inner tube stock or natural rubber (NR).
S5. Requirements. This section specifies performance requirements for DOT 3, DOT 4 and DOT 5 brake fluids; requirements for brake fluid certification; and requirements for container sealing, labeling and color coding for brake fluids and hydraulic system mineral oils. Where a range of tolerances is specified, the brake fluid shall meet the requirements at all points within the range."

In accordance with the above standards, all brake fluids that meet these criteria are non-mineral oil based (non-petroleum) by specification. Common compositions may include synthetic glycol ethers, borate esters, or silicone.

DOT3, DOT4, SUPER DOT 4, DOT5, DOT5.1 ...ALL of these fluids are tested for compatibility with all common automotive seal types, including natural rubber.

Anyone interested in reading the entire Federal Standard #116(or has the stamina to do so) can find it at the following web site:
Cheers - Dave

David DuBois

Hoo boy! I see this old chestnut has popped up again. OK, here goes-

Amongst the brake fluids presently available there are three possible candidates. The first, DOT 3, is a poor choice for high performance driving due to its low wet boiling point of F284 (dry boiling point F401) and is now generally considered to be obsolete. Be advised that American DOT 3 brake fluid is formulated differently than British DOT 3 and is incompatible with the Original Equipment natural rubber seals used throughout the brake system. It will slowly but surely dissolve them! However, after all of these years the original seals have long been replaced with seals that are made of more modern materials. The second candidate, DOT 4, is much better with a wet boiling point of (dry boiling point F446). Of the different Brands of DOT 4 brake fluid on the market today, Castrol LMA appears to be the best. The third choice is a synthetic fluid, Valvolene SynPower, which wins with a wet boiling point of F343 (dry boiling point of F513). DOT 5 Silicone-based brake fluid is a poor choice for any automobile as it has problems with air retention, making bleeding of the brake system a real bear, and poor lubrication, sometimes allowing the pistons of calipers and/or slave cylinders to bind in their bores and lock up the wheels. With a boiling point of F500, its performance is inferior to that of Valvolene SynPower. While it is true that silicone-based brake fluid does not absorb water, water still gets into the system through condensation. Because water is heavier than silicone fluid, it will ultimately sink and gather in the lowest point in the system. Should it freeze, line blockage and brake failure becomes possible. It is also possible that should the temperature of the brake fluid rise above 212F, the water will vaporize, increasing pressure within the system and locking a brake. Brake systems with small orifices or rapid action of the system, such as automatic proportioning valves and antilock braking systems should definitely not use silicon fluid since the small orifices and rapid operation of the system will cause airification as a result of cavitation of the fluid, which will at beast certainly cause a spongy pedal. Should you decide to use silicone-based brake fluid, be sure that all of the seals are in excellent condition as it will easily find its way past a leaky seal and air will get into the system. Be sure to flush the system with denatured alcohol prior to refilling it with the silicone fluid. Failure to do so will result in the residual glycol-based fluid interacting with the silicone fluid to form a sludge that will destroy the seals in the system, resulting in catastrophic brake failure. Perhaps the only argument in favor of silicone brake fluid is that it is slow to damage paint. However, if the brake system is leaking, it should be promptly repaired in the interests of safety, regardless of the type of brake fluid used in it.
Steve S.

I did not know that the natural rubber has been changed.
In the 60, 70 and 80 I have seen dot 3 destroy many braking and clutch system. I had to learn the hard way. You can ask an Auto parts store to carry Castrol dot 4. I know someone who got the auto parts store in Tacoma WA on 84th and south tacoma way to carry it. live and learn

I see disavantages of silicone for brakes...but what about for clutch hydraulics assuming the system is bled perfectly....boiling point should not be an issue...Can water get into the clutch system or is it a sealed system...

I have used the Valoline SynPower fluid, that Steve mentions, for almost a decade now in both the MGA I once owned and in the 68 GT I still have. In both cases I never had a bit of trouble with this fluid in either car. One thing I have noticed is that this brand of fluid tends to stay clear, if a little discolored over time, in both car's systems, Girling and Lockheed. This indicated to me that unlike ordinary fluids used in other of my vehicles when I got them, it did not seem to react nearly as strongly with the seals, thereby not creating as much debris in the fluid to make it turn opaquely black. That is just an empirical observation, but significant in my experience.

As for damaging paint, I have found that the Valvoline fluid, if cleaned up almost immediately with a Simple Green type degreaser solution and then rinsed, seems to do little if any damage as compared to more ordinary formulations. Some of them damage the paint almost immediately to a point where the paint lifts seemingly in seconds after contact. It probably has something to do with the choice of glycol esters used, but this slower reactivity has been greatly appreciated by myself. FWIW
Bob Muenchausen

I was using DOT3 on my 76 MGB and was always having problems. Then I read somewhere that there is a difference between American DOT 3 and British DOT 3. The American DOT 3 was eating the rubber of the brake system. I switched to Castrol LMA 4 years ago and have not had a problem since.
Paul Ronning

>Can water get into the clutch system or is it a sealed system...

Glycol-based fluids are extremely hygroscopic (moisture-absorbing) and will suck water out of the air straight through the pores of the rubber hoses. Silicone fluid is not hygroscopic and will not pull in moisture from the air.

As per the FMV standard Dave cited, D.O.T 3 fluid *IS* compatible with natural rubber, so it won't hurt it. However, for the boiling point characteristics Steve mentions, it's not the ideal choice. In a pinch, though, it won't hurt anything....

Rob Edwards

Yesterday I emptied my master cylinder by spoon and replaced the dot3 with dot4 fluid. At least this keeps the amount of dot3 to a minimum until I have a chace to bleed the system. The master cylinder had been replaced about ten years ago so I looked at the cap. sure enough the cap said to use dot3! Perhaps the newer cylinders are made to accept dot 3 now. I'm still going to bleed it. Thanks alot. Bob
Bob Ekstrand

As far as the clutch goes, a little contamination will take a LONG time to kill that system. Principal cause of death in clutch hydraulics is plain old mechanical wear of the seals.

Pressures and temperatures in the clutch hydraulics are really nothing compared to the brakes, so boiling point means nothing. You could run the clutch acceptably on maple syrup, so long as you change it before those little crystals start forming around the top of the bottle...
Sam Good

BTW, Castrol LMA has a new formulation, fully synthetic. It's minimum wet boiling point is now 329F (up from 311F for the old style LMA). Not quite up to the Valvoline, but better than everything else. They had both old and new on the shelf yesterday.

Bob, every Pep Boys in town carries Castol LMA! Give me a call, I have a couple of cars I need to flush and bleed, we might as well do yours at the same time.

Paul K

This thread was discussed between 12/03/2008 and 23/03/2008

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