On another forum the discussion of brake fluid and merits of DOT 4 and DOT 5 as an alternative to DOT 3 promts this post I hope will benefit some if not all.
DOT 3 glycol is highly hygroscopic...it loves to attract water. Keeping the water in suspension the water tends to do most of its damage at the wheel cylinders...especially at the rear.
Castrol LMA (low moisture attracting) is a synthetic which actually exceeds DOT 3 and 4 standards. While slightly more expensive the concept of longer life makes it a good bargain and better alternative. I have been selling it and many years to my British Car customers and its the only brake fluid I recommend and vend. Its reportedly easily available on the two coasts at better auto supply stores or from me in either 12oz or 1 qt plastic bottle
DOT 5 is silicone which has benefit to some in that a spill will not harm paint. However, due to the tiny molecular structure is is always and forever air embedded...spongy pedal. The slight aeration in the master cylinder serves to add more air all the time. As the master has a vent hole water is also drawn in via the atmosphere itself. Silicone will NOT absorb the moisture and thus it collects in the system much the same as DOT 3 moisture settleing into the lower areas and causeing errosion/rust.
Thus its recommended that a purge be performed at least once every two years for the glycol based fluids (including LMA) and annually for silicone fluids where both water and air need to be exsponged. In the case of silicone a complete bleed and replacement is advised. For DOT 3/4 enough to purge the wheel cylinders themselves should be sufficient.
I'm rather confident this flys in the face of most reading here but the eventual problems with rust in the cylinders make this advised procedure a rather easy and profoundly better idea than ignoreing what moisture trapped in your system can eventually lead to, total slave replacement. Its most important to the rear slaves as they tend to be the first to suffer based on my many years of maintaining many vehicles.
I hope these suggestions will be of benefit.