n.b.
For those of you spouting that 5.1 is 'better' and how you are gods gift to mechanics because you bleed your brakes with a fresh bottle of 5.1 every other week?
You do know why 5.1 is 'better' dont you?
So why bother replacing it so often?
For the rest of you………..
DOT3, DOT4 & DOT5.1 are all basicaly the same chemicals, but with varying levels of addatives to keep the boiling point consistent as they absorb water. The side effect of these addatives is to drop the boiling point of the 'dry' fluid. So if you do in fact change the fluid every 5 minutes, you'd be better off with a good quality (yes not all brake fluid is created equal) DOT4. The DOT standards are a minimum the fluid has to meet. So yes DOT4's standard is a lower boiling temperature, but a good quality DOT 4 will be significantly higher when dry, it just drops off quicker as it absorbs moisture from the air.