DOT 3 and 4 are glycol-based fluids, which absorb water ("hygroscopic"). DOT 5 fluids are silicon based, and while they don't absorb water, they are incompatible with many older brake component rubbers, and also with glycol based fluids (they can't be mixed).
The merits of converting to DOT 5 fluid was discussed at length on the earlier version of this site, and the consensus (if there was one) was that DOT 5 fluid should ONLY be used if ALL the brake system rubbers (including wheel, disc and master cylinder seals, & all flex hoses) had been replaced with new (not NOS) components. It was also imperitive that the old fluid was thoroughly flushed out by using butyl alcohol in the lines.
The problem with water in brake fluid is two-fold. First, it can cause premature corrosion of the components, especially caliper and wheel cylinder bores. Secondly, when glycol based fluids gets saturated with water, the boiling point of the fluid gets reduced dramatically. Heavy braking boils the fluid and creates steam, which is "compressible" (versus the fluid itself which is incompressible) and the brake pedal gets soft, or even ineffective. This is really only a concern among the racing crowd, since it is VERY rare (unless you do a lot of mountain driving) to get on the brakes this much.
Glycol based fluids, if changed every two years, will perform very satisfactorily for everyday use.
An earlier post gave this address:
http://www.shotimes.com/SHO3brakefluid.html
In it, one of the writers suggest that while silicon fluids don't directly absorb water, they have the disadvantage that if water DOES get into the system, it will "pool" and cause excessive corrosion. It also lists the "dry" and "wet" boiling temperatures of various fluids.
HTH
The merits of converting to DOT 5 fluid was discussed at length on the earlier version of this site, and the consensus (if there was one) was that DOT 5 fluid should ONLY be used if ALL the brake system rubbers (including wheel, disc and master cylinder seals, & all flex hoses) had been replaced with new (not NOS) components. It was also imperitive that the old fluid was thoroughly flushed out by using butyl alcohol in the lines.
The problem with water in brake fluid is two-fold. First, it can cause premature corrosion of the components, especially caliper and wheel cylinder bores. Secondly, when glycol based fluids gets saturated with water, the boiling point of the fluid gets reduced dramatically. Heavy braking boils the fluid and creates steam, which is "compressible" (versus the fluid itself which is incompressible) and the brake pedal gets soft, or even ineffective. This is really only a concern among the racing crowd, since it is VERY rare (unless you do a lot of mountain driving) to get on the brakes this much.
Glycol based fluids, if changed every two years, will perform very satisfactorily for everyday use.
An earlier post gave this address:
http://www.shotimes.com/SHO3brakefluid.html
In it, one of the writers suggest that while silicon fluids don't directly absorb water, they have the disadvantage that if water DOES get into the system, it will "pool" and cause excessive corrosion. It also lists the "dry" and "wet" boiling temperatures of various fluids.
HTH
