JOINING THE DOTS
QI’ve just rebuilt a 1990 Land Rover Defender and renewed all of the brake components. I now need to fill it with fluid, but which DOT standard should I go for? I want it to last and obviously work well! Garry Holmes, Ashford
Ed says:
ADOT 3,4 and 5.1 are all ester-based fluids. DOT 3 is the oldest spec with the lowest boiling point and 5.1 is the latest, with the highest. DOT 5 is siliconebased, with a boiling point between 4 and 5.1. DOT 3 is regarded as more friendly to older formulations of rubber seal than 4 or 5.1, and DOT 5 silicone is regarded as more-or-less harmless to everything, including seals designed for the now-extinct castor oil based fluids of the Sixties and before. As you’ve rebuilt the entire system with new parts, you could use any of the above fluids, but we’d recommend DOT 5 silicone most strongly. It repels water, where the others tend to attract it. This point is particularly important in cherished cars that may not be in daily use, or be parked over the winter. There is always a film of brake fluid left behind on cylinder walls as pistons retract and it’s this that promotes rusting and seizure, as the fluid assimilates moisture in the atmosphere.
Silicone doesn’t do this, so the time between brake repairs is greatly increased.
ABS systems are said to work better with ester-based fluids, although we’d certainly welcome any official clarification of this theory from a silicone fluid maker or distributor.