So I'm about to replace the MC in the 1993 Honda Accord LX 4-cyl 2.2L when I realized that I have no idea what kind of brake fluid is already in the lines.

I've read an ominous warning about mixing brake fluids which can cause serious issues. At this point I'm not sure what to do. I'm thinking I have to bleed all of the brake fluid out? But that doesn't guarantee there isn't some residual fluid left to mix with the new fluid? I was planning on replacing with DOT 4.

The lines are definitely filled with old, probably moisture and debris filled fluid. I'm looking to get all of that out of the system. While I'm here, does anyone swear by a brand or type of brake fluid and want to recommend one for me?

 

Search trotrlIG2sk on YouTube.

This is a video on how to change the MC and bench bleed it. This is for an integra, but it should be very similar to your accord.

As for fluid, I would go with whatever is recommended on the MC cap or in the owner's manual.

After installing the MC, I would bleed the fluid at each brake caliper. If you use a clear hose, you should be able to see when the clear fluid starts coming out.

 

DOT-3, DOT-4 and DOT-5.1 can be mixed, especially if you're only talking about some residual left mixing after you bleed the whole system.

DOT-5 is silicone fluid (not to be confused with DOT-5.1) and silicone fluid WILL NOT MIX with others. In fact, DOT-5 is NOT a good idea to use in your car anyway.

DOT-4 has higher boiling points than DOT-3, but DOT-4 tends to absorb moisture & crud more quickly. So if you're planning to refresh the fluid every couple years, DOT-4 is good. If you're gonna leave it for 4 years, then go with DOT-3.

At different times I've used different fluids & haven't had any problems with them...
Castrol LMA
ATE Super Blue & Gold (same stuff different color so you can see when the new stuff comes out)
ATE SL6