Of the DOT 3 fluids, the Ford fluid is one of the best. I don't know why it is not rated as a DOT 4 fluid. I have heard some Ford engineers claim that it meets DOT 4 specs although I doubt it. For a DOT 3 fluid it has one of the highest dry boiling points (550 F).
Of the DOT 4 fluids, two of the best of the commonly available fluids are Castrol LMA and Valvoline's synthetic. Dot 4 fluid has a higher affinity to absorbe moisture than DOT 3 and as a result must be changed at least every two years. The LMA in the Castrol LMA fluid means low moisture absorbption. Castrol claims it is more stable than some other DOT 4 fluids. Valvoline also claims to have a superior fluid.
You may ask why is moisture a problem when the brake fluid is contained in a sealed system. There is no such thing as a perfectly sealed system. In normal operation with large changes in temperature and pressure, some minute amount of air with whatever moisture is in the air will get past the seals. In time the added moisture causes the fluid to degrade. That is why the specs you may find on the container provides both wet and dry boiling points. The wet boiling point is when the fluid is saturated with moisture.
Besides lowering the boiling point, the moisture in the fluid can cause damage to brake parts. It will allow internal parts to corrode and cause the metal to become pitted. It will also cause a sludgy build up that will eventually interfear with the operation of cylinders and calipers. It will also wreak havoc with ABS hydraulic controls which have very small orfices to function the ABS system.
With all that said, I use Valvoline's synthetic DOT 4 fluid and I change it once a year. By using a premium fluid (and any of the three I mentioned are premium fluids) and changing it often I expect the hydraulic parts in my brake system to last at least 200,000 miles if not more.
By the way, Ford has no maintenance interval for brake fluid changes with DOT 3 fluid. In my opinion even Ford's DOT 3 fluid should be changed at least every two to three years. Like I said, though, I change mine every year.
Jim Johnson
98 SVT
