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MG MGA - DOT 5 or DOT 4 Brake Fluid?

I have read that DOT 5 (silicone) brake fluid can be a bit "squishy.". I use it and was thinking of changing to DOT 4. Anyone know of any issues with doing this?
(59 A with all drum brakes.)
Steven B


Periodically this type of question comes up. Many of us on this board love silicone fluid and won't have a word said against it - me included.

A word of caution if you do decide to swap over. The 2 types of fluid cannot be mixed. You will have to completely flush all the pipework and cylinders with something like acetone and replace every seal.

Steve Gyles

I went with DOT 5 in my B. I had heard all the problems with moisture absorption and rusting out brake lines and squishy pedal. Bottom line for me, no lifting of the paint when the fluid drips on it. When I finish up the 58 A DOT 5 is going in it. by the way, no squishy pedal. I see no downside to DOT 5

DH Cole

I have been using Dot 5 in my "A" since 1985 with good results. Steve is absolutely correct Dot 5 does not play well with others and you must get every bit of it out before the new stuff can go in which means tearing down the whole system. It is harder to get air bubbles out of Dot 5 so you many have to do more bleeding than with other types of fluid. Also, I have found that I have more success when the pedal operator on the bleed team uses slow steady gradual pressure in the up and down strokes rather than mashing the pedal to the floor as I was used to with other types. You should have a good firm pedal with this stuff. One other thought, how old are your break flex lines? They can get "squishy" and expand sideways when you step on the pedal and give you soft pedal feel.
Keith Lowman

This is one of those politics/religion questions. Lots of strong opinions here.
I use DOT 5 only, never a problem in 4 restored cars in the last 27 years.
I admit that I used paint thinner in my first MGA in 1975, but beg forgiveness, I was ignorant of a better way.
Praise Silicon

To use silicon you must:

See the error of your ways/ Paint thinner is evil
Repent/ promise never to do evil again
Cleanse the inner vessel/ thoroughly cleaning all
Go forth in purity/using new seals and fresh fluid
With an eye single to silicon.
R J Brown

Looks like the nay sayers are pretty quiet! Since I redid all the seals when I bought the car, I'm not wild about redoing them. Have enough unfinished projects already!
Thanks for the feedback.
Steven B

I have had good luck with DOT 5.1 (NAPA) in my "A."
David Werblow

I am another DOT 5 fan. I first used in my midget in 1989 and have been using ever since. I put in my MGA in 2006, together with all new seals and hoses. I didn't flush the system other than an extra litre or so during bleeding. A couple of years ago one of the guys mentioned that there was some discolouration in his fluid and others have suggested that water may be an issue. To check I flushed a jar full of fluid through each slave and the fluid looked exactly as it did when it went in. I have a good firm pedal and I had no problems bleeding with Eezibleed.

I have a couple of classics that I have not switched over yet and so I do use DOT 4 too.

N McGurk

I see this old chestnut has returned. Looking forward to some lively debate!
Graham M V

This article is now being uploaded for member's perusal on the MGCC website. It dates back to 1999 and, in my opinion, makes a good read:

"Foremost among the problems of conventional glycol fluids, is their hygroscopic properties - when exposed to air, glycols absorb up to 12% water by volume. The water lowers the boiling point. Under normal driving conditions, a car, driven daily, or even weekly, will heat up the brake fluid and evaporate a significant amount of the absorbed water. At higher percentages of water the brake cylinders corrode rapidly leading to early brake failure (probably due to electrolytic action as the mixture conducts more). This occurs in cars that have not been driven for several months. On being driven, the rust or corrosion soon causes one or more seals to fail.

The other effect, lowered boiling point, does not usually bother the ordinary driver. It does, however, cause problems under extreme braking conditions, such as in racing at Le Mans where the brake discs can be seen to glow in the dark. Silicones were first used at the 12-Hour Endurance Race in the sixties on the Chaparral, an innovative ground effects race car. Most club racing never approaches this kind of heat load from braking.

The U.S. Army undertook a long term study of the problems of brake failure, primarily due to-corrosion, extreme heat (desert conditions), and extreme cold (arctic) conditions. They were experiencing an enormous maintenance problem. The average Army vehicle logs only about 2000 miles per year, with most of the mileage occurring in short bursts followed by long periods of inactivity. They found that, invariably, vehicles that had been sitting for several months had brake failures after being driven a few miles. They then evolved an elaborate scheme of draining, flushing and refilling the brake system with a non-hygroscopic preservative for storage, and then a similar reverse process for getting the vehicle ready for use. This was expensive and delayed 'readiness' but cost less than the continual overhauling of brake systems.

After extensive study they found that silicone fluids (DOT5) could be used instead. All U.S. Army vehicles with hydraulic brakes now use silicone with an attendant savings of in excess of $20 million per year over the flush-refill protocol. An additional benefit was the wider range of temperature extremes the silicone had over glycol based fluids. The U.S. Postal Service also adapted silicones for the same reasons.

Silicones do not attack natural rubbers and most synthetic rubbers and vinyl, and are often used as rubber vinyl preservatives (as in many car care products). In addition, it does not attack auto paints! The earlier U.S. version of D0T3 would destroy the natural rubber used in early Lockheed and Girling brakes from England. Only Girling "Crimson" and Lockheed "genuine" fluids were usable, and they were hard to get in the US. The later US D0T4 specification seemed to overcome this, as did the change to synthetic rubber for the seals. CASTROL-LMA (Low Moisture Absorption) did better than most.

DOT5 silicone fluid does not attack British made seals. An exception seems to be with some German manufactured synthetic seals (such as lately used on Mercedes). These compounds do react with silicones and more readily with ozone (the sidewalls of tyres seem to age more rapidly than those manufactured in other countries).

I have used DOT5 Silicone brake fluid in my 1962 MGB (both clutch & brake) since 1984 and in my TF 1500 since 1986. In both cases, I have added fluid only once since (a slight topping off). I have had no hydraulic problems with either car. On the other hand , I have had to replace or rebuild cylinders on my other cars filled with DOT4 glycol based fluid, invariably after the car was laid up for other reasons.

When I switched over to silicone on the MGB, I first attempted to simply flush of the system by adding silicone and bleeding the brakes. This sounds like it should work since the two don't mix (are immiscible), and was the method suggested by the supplier. I found, however, the following:- the old brake fluid contained a lot of water, which in contact with the silicone, separated into three layers: water, silicone, glycol. Ordinary bleeding procedures, designed to remove air, left water in the bottom of the wheel cylinders. A sure invitation to corrosion and failure in time. However the brakes work fine at first, since any "non-compressible" fluid or mixture will work. (See the Army emergency procedures for loss of coolant or hydraulic fluid under combat conditions, using readily available fluid from every soldier in the unit. Note that a complete overhaul is then necessary to minimise corrosion afterwards).

I have found that the best way to switch over is with a complete system overhaul. The brake lines should be flushed with acetone which dissolves the glycol gummy residues and removes any trapped moisture. The lines should be dried by blowing with air (ethyl alcohol can also be used, it is not as flammable and won't injure paint). All flexible lines and seals should be replaced so that all rubber that has been exposed to the glycol is removed. I think that the problems some have had with silicone may arise from not doing this. It may be that the problems come from the interaction of the different swelling agents in the silicone with those used in the glycol based fluid.

Some time ago, an article appeared about the dangers of using silicone. It was written by (I believe) a sales engineer for a company who markets DOT4 type fluid. The myths created by this have been widely quoted since. Among the myths is the fact that silicone is 3-times as compressible as glycol (TRUE) and that this leads to excessive pedal travel, such that the pedal will travel as much as 3cm further (FALSE). While it is 3 times as compressible, the compressibility is still a very small number. With the volume of our brake systems, the additional compressibility would at most add 0.1mm to the pedal travel. Most of the problems with "soft" pedal arise from air bubbles entrapped, and poor bleeding. One should take the precaution of pouring carefully so as not to introduce air bubbles. Wait for 10-15 minutes for any bubbles to escape, and then bleed the brakes."

Steve Gyles

I use both types of fluids in my cars, and neither has ever been a problem. I run silicon in the MGA, both MGBs and MGC. The pedal may be very slightly softer with silicon but that's the only downside (other than higher price and harder to find) that I've noticed. The worst part about Silicon fluid is bleeding. It's definitely harder to do properly than DOT 3/4. Pour gently, never shake up the bottle, and bleed again after a few days. Once the first bleed is over with, life is grand. And the biggest advantage in an MGA is as others noted above - no more peeling paint on the front shelf!
Steve S

I've had dot 5 in my '57 MGA for 10 years with no problems other then what already has been said.
I was replacing wheel cylinders in my '57 Ford and '62 T-bird every couple years. I flushed the systems with alcohol and now use dot 5 with no problems.
Lyle Jacobson

I know I didn't do an acetone flush so I might have some swollen hoses. I'll take a look at that.
Thanks for the great feedback!
Steven B

This thread was discussed between 09/01/2012 and 12/01/2012

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