Gasket genius: We need you now!

The DOT5 fluid is a good choice if you don't want the paint hurt. That's DOT5, not DOT5.1. They're different.

Also, it's not good to mix DOT5 with DOT4 or DOT3 as the seals will swell and become useless. So that means you either have to flush the HECK out of the system, OR you could replace all the rubber parts. Kinda a pain if the system is already together.

It is also true DOT5 does not absorb moisture. That does not mean moisture doesn't get in the system. It does. It just does not mix with the fluid. So, it gathers "wherever it is" and accumulates. I'm not sure which I prefer in that situation.

I've always used DOT3 and just "dealt with it". But you're right. I too have never seen a master cylinder that's completely leak free. One reason is, they must have a vent in the cap "somewhere" and that normally is where the small amount of fluid comes from.