i am getting ready to bleed my brakes and switching to dot 4 (i know it compatable w/ dot 3). should i drain all the dot 3 first or just add and bleed.

i lost a good amunt of fluid while installing the baer rear conversion already.

It's really up to you. The drain+fill is going to take twice as long to get all the bubbles back out though.
Add+bleed is what most people will recommend.

Bleed and add is perfectly acceptable.

And for anyone else wondering, DOT5 can NOT be used in our systems.

so DOT3/4 is compatible

DOT 5 is not

what about DOT 5.1 ?

14.82 @ 97 mph

That's correct.

And I've never heard of DOT 5.1 but from the name, I'd infer that's it's likely compatable with DOT 5 and, thus, not with our systems. But you'd have to look into it since I really just made that up.

Just stay away from DOT 5, even 5.1 or anything else with a 5 in it.

It's silicone based and will not absorb water, which is a very bad thing in our cars.

DOT 3 is good enough for anyone normally.
DOT 4 is very good stuff.

DOT 5.1 is DOT 5 with additive to make it "compatible" with the parts of our system, otherwise it would eat our system. But it still doesn't absorb water, so you will end up with water in the lines, when you use the brakes it will steam, and you will have steam bubbles in your brake lines. Anyone should know that/why this is a very bad thing. Don't use it in a J-Body!

Not only that, but DOT 5 is slightly compressible as well. Definitely not something you want in our cars.