What are the pro’s and con’s about using silicon brake fluid? In an old car that sits still 95% of it’s life, would it be a good thing to have in the system? Car is currently being restored and the brake system is going to be flushed anyway.

The good: It doesn't absorb moisture (read less rust), it won't melt your paint if you spill it, it also supposedly compresses less so you get a firmer pedal. It may also have higher operating temps (not sure on that one).
The bad: It's expensive, if you need to top off when you're on the road it can be tough to find at a service station, it's a PITA to thoroughly purge all your lines of the dot 3 when converting.
I think it's worth the investment for cars that don't see a lot of miles each year.

I run it in my Dart with no problems. I have heard it can be a little more difficult to blead as it can have micro bubbles in it that are hard to detect. But all good with mine.

I am completely rebuilding the brake system from master cylinder back except for the steel lines. This includes new rear wheel cylinders, front calipers and rubber hoses troughout. Would compressed air sufficiently clean out the steel lines of old dot3 if I chose to switch to silicon? Or should I flush with alchohol or something else?

Blowing out the lines with air will be sufficient,
a small amount of residual DOT3 fluid won't cause any problem when converting to silicone.

You can drain and flush out with denatured alkie,or you flush with dot5 till u think its good.I have it in both cars and the pedal is firm and good.

You will hear the naysayers say all the bad things like water will collect in the lines and rot a hole through the line, among other things.

I have been using it exclusively for the last 20+ years on all my own and the cars I build for others with NO issue. All it takes is for one drop to ruin a freshly painted car. , and you will never use the DOT 3 again. Conversely, I am not proclaiming that you need to purge every car you own and put the silicone fluid in, just that the special cars, restorations etc. should use the stuff. Works great on the tires, too.

MB

Exactly, all you need is to have paint ruined one time....just one time. And if your running new lines or lines that have been reused. A leak before tightening is not uncommon. I wouldnt use anything in a restored or newly painted engine comnpartment or bottom but silicone. Remember just one time is all it takes.

The post office has been using it for years in the older mail delivery vehicles.No problems with it ever and it does not harm anything or collect moisture.

Silicon has been in my car for 10 years---no problems.