To use or not use DOT5 fluid?
The info about silicone fluid (DOT 5) being incompatible with glycol fluids (DOT 3, 4, 5.1) is unfounded. It is a long-spread rumor by most auto parts guys, but that doesn't make it true. I even read that warning on a bottle of DOT 5, as I recall. They do not react, and indeed do not even mix. I read a report where it was investigated by engineers and found untrue. The only problem with leaving some glycol in the system is that those are areas that can rust. I have always been changing hoses and wheel cylinders when changing to DOT 5 and cleaned all the tubes with alcohol then air first, so never even concerned me.
All brake fluids are synthetic, whether silicone or glycol. Always have been. Some marketing genius figured out they could sell more by adding "synthetic" on the name (I think Valvoline).
The military uses DOT 5. Indeed, mil surplus is the best source for less cost, if you buy a gallon. I just replaced the MC in my 65 Newport to change to a dual system one. I installed it w/ DOT 5 in 1992, and it looked new inside. No more rusty fluid or pitted wheel cylinders since I changed, and no more need to bleed the fluid regularly.
The main drawback of DOT 5 is that it is more compressible, so you get more pedal travel. If interested, here is a link to a detailed analysis by an anal engineer with a lot of time on his hands, a requisite for an owner of classic British cars:
www.buckeyetriumphs.org/technical/Brakes/Fluid/Fluid.htm
I installed a 7/8" D MC in my Newport, compared with the 1"D factory one, and the pedal is quite stiff after I did the "back up and stop several times to set drum adjusters". I can skid the tires by pressing the pedal <25% of the way to the floor. I have never had any trouble with bubbles. Just pour it slowly down the side of a funnel, and if you drop the bottle let it sit for a few hours.
Don't get fooled by DOT 5.1. It is the first glycol fluid to meet the higher DOT 5 rating. The government makes them call it "5.1" to not confuse it with silicone. Think that works?
Finally, nobody mentioned the main problem with glycol fluids absorbing water. Sure, it rusts your brake parts and can pit cylinders and make them leak, which eventually makes your brakes fail. But, before that, absorbed water greatly decreases the boiling point, which can make your brakes totally fail, i.e. your pedal goes to the floor and no brakes. If worried about rust, note also that when glycol removes the paint around the MC, you get a lot of rust on the sheet-metal, which is typical on most old Darts. If still scared of DOT 5, or can't afford it, at least flush your glycol fluid every few years and quickly clean up any spills on the paint.