Rust in wheel cylinders

[JimKshortstep4x4]

Rust occurred in the rear wheel cylinders from the time I rebuilt them until the the truck was on the road, (approximately 4 months). They were on the rear end which was assembled with the wheel cylinders/brakes/drums etc. installed. This is a first for me, having rebuilt hundreds of them.

The procedure that I use is pretty standard, clean, hone with mineral spirits as a cutting fluid, clean with mineral spirits, (more than one time), clean with alcohol, and clean with brake fluid.
Assembly is done using liberal amounts of brake fluid, (Prestone).

I am concerned that there is a problem with the brake fluids anti-corrosion properties and I am looking for experienced suggestions or ideas. Are there better corrosion resistant fluids that you have had experience with?

Any input would be appreciated.

Jim