Pull to the left fixed

[Minn-Tex Lee]

Hydraulic brakes, 1800 miles, never driven in the rain, brake lines blead, shoes properly adjusted, DOT 3 fluid, new Cling set up.
For quite some time I have experienced a sharp pull to the left when I hard stop. Doing everything I could think of with no solution, I finally replaced the two front cylinders. That fixed the problem.
When I got back from my successful test ride I examined the old cylinders.
There was a lot of "crud" on the outside of the pistons, the left rear piston was frozen in place, both the left and right were rusty inside and out.
My question is: could this be from condensation, since I never have had it in the rain.
I sure don't want to go thru this again if I can help it.
Thanks
Lee