Originally Posted by nsxtasy

The short answer:

You can go ahead and mix or flush (replace) any DOT 3, DOT 4, or DOT 5.1 brake fluids, no problem. So there are no special bleeding procedures and you don't need to run anything in the lines. The only one to watch out for is DOT 5, which neither ATE nor Motul RBF 600 are. And neither ATE nor Motul RBF 600 will destroy the seals on your master cylinder.

The long answer (Brake fluid basics):

1. The boiling point of brake fluid is the temperature at which it boils. The higher the boiling point, the better the protection against the brake system boiling and losing effectiveness.

2. Brake fluid is hygroscopic, which means that it likes to absorb water. When brake fluid is put into the system (from an unopened container), it is dry, meaning that there is no water in the brake fluid. At this time, its boiling point is at its highest - the "dry boiling point". Over time, the brake fluid will absorb moisture from the air, which lowers its boiling point, until it reaches the point when it has absorbed all the moisture it can. At this time, its boiling point is at its lowest - the "wet boiling point". Both boiling points are important in evaluating the protection that a brake fluid gives you; the dry boiling point may be more relevant if you flush your fluid frequently, while the wet boiling point may be equally important if you don't.

3. The Federal government Department of Transportation (hence the initials DOT) has standards for brake fluid. These standards include minimum temperatures as well as composition and other criteria (color, performance, etc). If you're interested, you can read them yourself: Look up TITLE 49, PART 571, Subpart B, Sec. 571.116, Standard No. 116; Motor vehicle brake fluids, on the GPO (Government Printing Office) website.

4. DOT 5 brake fluid is silicone-based brake fluid (SBBF). There is also a standard for DOT 5.1 brake fluid, which is non-silicone based brake fluid with the same minimum boiling points as SBBF.

5. You can mix DOT 3, DOT 4, and DOT 5.1 fluids with each other without any problem. You cannot mix any of these non-SBBF fluids with DOT 5 SBBF. You have to completely flush the brake fluid if you change to DOT 5 fluid, because DOT 5 SBBF cannot be mixed with DOT 3 or DOT 4. But you can use DOT 5.1 non-SBBF without any problems. The main reason that Honda recommends DOT 3 or DOT 4 and not DOT 5.1 is because they don't want to chance the possibility of anyone trying to mix DOT 5 SBBF with non-SBBF.

6. A brake fluid may have boiling points that are higher than the minimum specified by the DOT regulations. There are fluids that are considered DOT 3 or DOT 4 but have boiling points that are higher than the DOT 5 minimums.

7. Here are the dry and wet boiling points for many of the most popular brake fluids:

ARRANGED BY DRY BOILING POINT:

DRY:401F -- WET:284F --- DOT3 MINIMUM
DRY:446F -- WET:311F --- DOT4 MINIMUM
DRY:500F -- WET:356F --- DOT5 MINIMUM
DRY:502F -- WET:343F --- VALVOLINE SYNPOWER
DRY:509F -- WET:365F --- MOTUL 5.1
DRY:527F -- WET:302F --- AP RACING 551
DRY:536F -- WET:392F --- ATE SUPERBLUE/TYP200
DRY:550F -- WET:284F --- FORD HEAVY DUTY
DRY:590F -- WET:410F --- AP RACING 600
DRY:590F -- WET:518F --- CASTROL SRF
DRY:593F -- WET:420F --- MOTUL RBF600
DRY:610F -- WET:421F --- NEO-SYNTHETIC SUPER DOT 610
DRY:620F -- WET:425F --- COBALT SUPER XRF

ARRANGED BY WET BOILING POINT:

DRY:401F -- WET:284F --- DOT3 MINIMUM
DRY:550F -- WET:284F --- FORD HEAVY DUTY
DRY:527F -- WET:302F --- AP RACING 551
DRY:446F -- WET:311F --- DOT4 MINIMUM
DRY:502F -- WET:343F --- VALVOLINE SYNPOWER
DRY:500F -- WET:356F --- DOT5 MINIMUM
DRY:509F -- WET:365F --- MOTUL 5.1
DRY:536F -- WET:392F --- ATE SUPERBLUE/TYP200
DRY:590F -- WET:410F --- AP RACING 600
DRY:593F -- WET:420F --- MOTUL RBF600
DRY:610F -- WET:421F --- NEO-SYNTHETIC SUPER DOT 610
DRY:620F -- WET:425F --- COBALT SUPER XRF
DRY:590F -- WET:518F --- CASTROL SRF

As you can see, some of these fluids have really high boiling points - much higher than the DOT 5 minimums - and the price on most (with the notable exception of the Castrol SRF) is quite reasonable.

You can order many of the better fluids from Andie at Cobalt Friction, who sells lots of great brake products at great prices, and can provide great advice on braking systems.