Its the damn salt. When I replace parts that tend to rust like this, I usually strip them to bare metal on a blasting cabinet then paint them in Zinc rich or weld through zinc primer, them some rustolem in any color.
As long as they part does not get the finish rubbed off this works really well.
For parts that like to flex (coil springs ect) I strip to bare metal and then spray them with plastidip. This is a rubber like coating that works awesome. IT looks great on trim pieces too and will hold up to some abrasion. Comes in red black and blue. By the can.
------ Follow up post added March 31st, 2011 09:49 AM ------
If you ever want to strip small parts ive got a blasting cabinet btw, feel free to come use it
ive got glass bead for alu and alu oxide for steel