At 01:27 PM 5/21/06, Michal Takac in Slovakia wrote:
"Which type of brake fluid is optimized for MGA?"

I have tried for years to avoid this question. If you ask this question on an open forum it will start a religious war where everyone is certain he is right and anyone else is wrong. The bottom line is, use whatever you like. If the system has been recently rebuilt the modern seals should be suitable for any commercial brake fluid.

When the cars were new in the 50's and 60's they used natural rubber seals, and many commercial fluids would disolve the natural rubber parts. The only prescribed fluid in those days was Girling fluid, commonly refered to as "the red stuff" or crimson fluid. It had a vegatable oil base which was compatible with natural rubber parts. I don't think anyone has produced natural rubber seals since the early 70's, and the crimson fluid has been generally unavailable since the early 90's. Do don't worry about it.

The commonly availlble fluids in North America are DOT3, DOT4, and DOT5.

DOT5 fluid is silicon oil, which is a natural preservative for all kinds of elastomer parts. For a restored classic car, the most endearing feature of silicone fluid is that it will not eat the paint off the body if the master cylinder happens to leak. The most significant deterent to using it would be the price, as it may cost over $20-USD per quart. But it will last a long time, will save maintenance cost in the long run, and is the only hydraulic fluid I would use in my MGA.

There is recently on the market a DOT-5.1 fluid which is not silicone, but has a glycol base more closely related to DOT4 type, but with a higher boiling point. This is probably expensive, if you can find it, but you have no need to use this in your MGA.

DOT3 is a glycol base fluid which has been used for almost everything American since it was so labeled in the 50's. The crimson fluid held on much longer in Europe. By the time Europe was ready for the change to synthetic rubber seals DOT4 was already available, which was then specified for mid-production MGBs. A little later the specification for MGB was changed to DOT3 fluid, probably as a cost cutting measure.

DOT4 is an uprated version of DOT3, rated for higher temperature. Certain versions of DOT4 fluid are also noted for low moisture absorbtion (such as Casterol LMA DOT4 fluid). The low moisture absorbtion characteristic increases the boiling point a little, and also helps to reduce corrosion of the metal parts if the fluid is not changed for a long time. When the crimsion fluid became unavailable it was customary to switch to DOT4 and to shun DOT3 fluid. This was probably because most of the European cars which previously used the red stuff had switched to DOT4 for new production cars, and people had an idea that DOT4 would be compatible with the earlier seals (which it is not).

People who are determined to use DOT4 usually cite problems with use of DOT5 fluid (generally imaginary problems). They might say that DOT5 fluid is compressible, which it is, sort of, but the compressibility is so minute as to be negligable. This in itself will not give the soft pedal feel which is so often quoted. They may also say that DOT5 fluid traps tiny air bubbles, which gives the soft pedal feel. The only precaution here is, don't shake the bottle, and pour slowly. It is very unlikely that you will ever have this problem, but if you do happen to get bubbles in the DOT5 fluid, let it set fot 12 hours before using it. If you have bubbles in the fluid in your car, let it set for 12 hours and bleed the system again. The DOT5 fluid does not retain air bubbles indefinitely, only for a short period of time, after which the bubbles rise to the top and disperse.

The nay-sayers may also say that DOT5 has the ability to sneak through very small spaces, which may lead to leaks which you may not have with other types of fluid. This is an "old wives tale", generally untrue. If your hydraulic seals leak while using DOT5 fluid the cylinder is about due for a rebuild anyway, and it may leak as well with any other fluid. The last argument against DOT5 is that it may not be quite as good a lubricant as other types of fluid, which could cause cylinder seals to stick and prevent return of the pistons. This is generally also untrue. People may get these ideas when a cylinder may leak or stick immediately after a cylinder rebuild when using DOT5 fluid. But in fact the same thing is just as likely to happen with other types of fluid.